Encoder, decoder, encoding method, and decoding method

ABSTRACT

An encoder includes circuitry and memory coupled to the circuitry. The circuitry, in operation: determines whether a size of a current block, which is a unit for which a vector candidate list including vector candidates is generated, is less than or equal to a threshold; when the size of the current block is less than or equal to the threshold, generates the vector candidate list by registering a history-based motion vector predictor (HMVP) vector candidate in the vector candidate list from an HMVP table without performing a first pruning process; when the size of the current block is greater than the threshold, generates the vector candidate list by performing the first pruning process and registering the HMVP vector candidate in the vector candidate list from the HMVP table; and encodes the current block using the vector candidate list.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/681,051, filed Feb. 25, 2022, which is a U.S. continuationapplication of PCT International Patent Application NumberPCT/JP2020/033171 filed on Sep. 2, 2020, claiming the benefit ofpriority of U.S. Provisional Patent Application No. 62/895,177 filed onSep. 3, 2019, the entire contents of which are hereby incorporated byreference.

BACKGROUND 1. Technical Field

The present disclosure relates to video coding, and particularly tosystems, constituent elements, and methods in video encoding anddecoding.

2. Description of the Related Art

With advancement in video coding technology, from H.261 and MPEG-1 toH.264/AVC (Advanced Video Coding), MPEG-LA, H.265/HEVC (High EfficiencyVideo Coding) and H.266/VVC (Versatile Video Codec), there remains aconstant need to provide improvements and optimizations to the videocoding technology to process an ever-increasing amount of digital videodata in various applications. The present disclosure relates to furtheradvancements, improvements and optimizations in video coding.

Note that H.265 (ISO/JEC 23008-2 HEVC)/HEVC (High Efficiency VideoCoding) relates to one example of a conventional standard regarding theabove-described video coding technology.

SUMMARY

For example, an encoder according to an aspect of the present disclosureis an encoder including: circuitry; and memory coupled to the circuitry.The circuitry, in operation: in generation of a prediction image of acurrent block, uses an intra block copy (IBC) mode in which a processedregion of a picture that includes the current block is referred to;determines whether a size of the current block is less than or equal toa threshold, the current block being a unit for which a vector candidatelist including vector candidates is generated; when the size of thecurrent block is less than or equal to the threshold, generates thevector candidate list by registering a history-based motion vectorpredictor (HMVP) vector candidate in the vector candidate list from anHMVP table without performing a first pruning process, the HMVP tablestoring, by a first-in first-out (FIFO) method, first vector candidateseach as the HMVP vector candidate, the first vector candidates eachhaving information regarding a first vector used for a processed block;when the size of the current block is greater than the threshold,generates the vector candidate list by performing the first pruningprocess and registering the HMVP vector candidate in the vectorcandidate list from the HMVP table; and encodes the current block usingthe vector candidate list.

In video coding technology, new methods are desired to be proposed inorder to improve coding efficiency, enhance image quality, and reducecircuit scales.

Each of embodiments, or each of part of constituent elements and methodsin the present disclosure enables, for example, at least one of thefollowing: improvement in coding efficiency, enhancement in imagequality, reduction in processing amount of encoding/decoding, reductionin circuit scale, improvement in processing speed of encoding/decoding,etc. Alternatively, each of embodiments, or each of part of constituentelements and methods in the present disclosure enables, in encoding anddecoding, appropriate selection of an element or an operation. Theelement is, for example, a filter, a block, a size, a motion vector, areference picture, or a reference block. It is to be noted that thepresent disclosure includes disclosure regarding configurations andmethods which may provide advantages other than the above-describedones. Examples of such configurations and methods include aconfiguration or method for improving coding efficiency while reducingincrease in processing amount.

Additional benefits and advantages according to an aspect of the presentdisclosure will become apparent from the specification and drawings. Thebenefits and/or advantages may be individually obtained by the variousembodiments and features of the specification and drawings, and not allof which need to be provided in order to obtain one or more of suchbenefits and/or advantages.

It is to be noted that these general or specific aspects may beimplemented using a system, an integrated circuit, a computer program,or a computer readable medium (recording medium) such as a CD-ROM, orany combination of systems, methods, integrated circuits, computerprograms, and media.

BRIEF DESCRIPTION OF DRAWINGS

These and other objects, advantages and features of the disclosure willbecome apparent from the following description thereof taken inconjunction with the accompanying drawings that illustrate a specificembodiment of the present disclosure.

FIG. 1 is a schematic diagram illustrating one example of aconfiguration of a transmission system according to an embodiment;

FIG. 2 is a diagram illustrating one example of a hierarchical structureof data in a stream;

FIG. 3 is a diagram illustrating one example of a slice configuration;

FIG. 4 is a diagram illustrating one example of a tile configuration;

FIG. 5 is a diagram illustrating one example of an encoding structure inscalable encoding;

FIG. 6 is a diagram illustrating one example of an encoding structure inscalable encoding;

FIG. 7 is a block diagram illustrating one example of a configuration ofan encoder according to an embodiment;

FIG. 8 is a block diagram illustrating a mounting example of theencoder;

FIG. 9 is a flow chart illustrating one example of an overall encodingprocess performed by the encoder;

FIG. 10 is a diagram illustrating one example of block splitting;

FIG. 11 is a diagram illustrating one example of a configuration of asplitter;

FIG. 12 is a diagram illustrating examples of splitting patterns;

FIG. 13A is a diagram illustrating one example of a syntax tree of asplitting pattern;

FIG. 13B is a diagram illustrating another example of a syntax tree of asplitting pattern;

FIG. 14 is a chart illustrating transform basis functions for eachtransform type;

FIG. 15 is a diagram illustrating examples of SVT;

FIG. 16 is a flow chart illustrating one example of a process performedby a transformer;

FIG. 17 is a flow chart illustrating another example of a processperformed by the transformer;

FIG. 18 is a block diagram illustrating one example of a configurationof a quantizer;

FIG. 19 is a flow chart illustrating one example of quantizationperformed by the quantizer;

FIG. 20 is a block diagram illustrating one example of a configurationof an entropy encoder;

FIG. 21 is a diagram illustrating a flow of CABAC in the entropyencoder;

FIG. 22 is a block diagram illustrating one example of a configurationof a loop filter;

FIG. 23A is a diagram illustrating one example of a filter shape used inan adaptive loop filter (ALF);

FIG. 23B is a diagram illustrating another example of a filter shapeused in an ALF;

FIG. 23C is a diagram illustrating another example of a filter shapeused in an ALF;

FIG. 23D is a diagram illustrating an example where Y samples (firstcomponent) are used for a cross component ALF (CCALF) for Cb and a CCALFfor Cr (components different from the first component);

FIG. 23E is a diagram illustrating a diamond shaped filter;

FIG. 23F is a diagram illustrating an example for a joint chroma CCALF(JC-CCALF);

FIG. 23G is a diagram illustrating an example for JC-CCALF weight indexcandidates;

FIG. 24 is a block diagram illustrating one example of a specificconfiguration of a loop filter which functions as a DBF;

FIG. 25 is a diagram illustrating an example of a deblocking filterhaving a symmetrical filtering characteristic with respect to a blockboundary;

FIG. 26 is a diagram for illustrating a block boundary on which adeblocking filter process is performed;

FIG. 27 is a diagram illustrating examples of Bs values;

FIG. 28 is a flow chart illustrating one example of a process performedby a predictor of the encoder;

FIG. 29 is a flow chart illustrating another example of a processperformed by the predictor of the encoder;

FIG. 30 is a flow chart illustrating another example of a processperformed by the predictor of the encoder;

FIG. 31 is a diagram illustrating one example of sixty-seven intraprediction modes used in intra prediction;

FIG. 32 is a flow chart illustrating one example of a process performedby an intra predictor;

FIG. 33 is a diagram illustrating examples of reference pictures;

FIG. 34 is a diagram illustrating examples of reference picture lists;

FIG. 35 is a flow chart illustrating a basic processing flow of interprediction;

FIG. 36 is a flow chart illustrating one example of MV derivation;

FIG. 37 is a flow chart illustrating another example of MV derivation;

FIG. 38A is a diagram illustrating one example of categorization ofmodes for MV derivation;

FIG. 38B is a diagram illustrating one example of categorization ofmodes for MV derivation;

FIG. 39 is a flow chart illustrating an example of inter prediction bynormal inter mode;

FIG. 40 is a flow chart illustrating an example of inter prediction bynormal merge mode;

FIG. 41 is a diagram for illustrating one example of an MV derivationprocess by normal merge mode;

FIG. 42 is a diagram for illustrating one example of an MV derivationprocess by a history-based motion vector prediction/predictor (HMVP)mode;

FIG. 43 is a flow chart illustrating one example of frame rate upconversion (FRUC);

FIG. 44 is a diagram for illustrating one example of pattern matching(bilateral matching) between two blocks located along a motiontrajectory;

FIG. 45 is a diagram for illustrating one example of pattern matching(template matching) between a template in a current picture and a blockin a reference picture;

FIG. 46A is a diagram for illustrating one example of MV derivation inunits of a sub-block in affine mode in which two control points areused;

FIG. 46B is a diagram for illustrating one example of MV derivation inunits of a sub-block in affine mode in which three control points areused;

FIG. 47A is a conceptual diagram for illustrating one example of MVderivation at control points in an affine mode;

FIG. 47B is a conceptual diagram for illustrating one example of MVderivation at control points in an affine mode;

FIG. 47C is a conceptual diagram for illustrating one example of MVderivation at control points in an affine mode;

FIG. 48A is a diagram for illustrating an affine mode in which twocontrol points are used;

FIG. 48B is a diagram for illustrating an affine mode in which threecontrol points are used;

FIG. 49A is a conceptual diagram for illustrating one example of amethod for MV derivation at control points when the number of controlpoints for an encoded block and the number of control points for acurrent block are different from each other;

FIG. 49B is a conceptual diagram for illustrating another example of amethod for MV derivation at control points when the number of controlpoints for an encoded block and the number of control points for acurrent block are different from each other;

FIG. 50 is a flow chart illustrating one example of a process in affinemerge mode;

FIG. 51 is a flow chart illustrating one example of a process in affineinter mode;

FIG. 52A is a diagram for illustrating generation of two triangularprediction images;

FIG. 52B is a conceptual diagram illustrating examples of a firstportion of a first partition and first and second sets of samples;

FIG. 52C is a conceptual diagram illustrating a first portion of a firstpartition;

FIG. 53 is a flow chart illustrating one example of a triangle mode;

FIG. 54 is a diagram illustrating one example of an advanced temporalmotion vector prediction/predictor (ATMVP) mode in which an MV isderived in units of a sub-block;

FIG. 55 is a diagram illustrating a relationship between a merge modeand dynamic motion vector refreshing (DMVR);

FIG. 56 is a conceptual diagram for illustrating one example of DMVR;

FIG. 57 is a conceptual diagram for illustrating another example of DMVRfor determining an MV;

FIG. 58A is a diagram illustrating one example of motion estimation inDMVR;

FIG. 58B is a flow chart illustrating one example of motion estimationin DMVR;

FIG. 59 is a flow chart illustrating one example of generation of aprediction image;

FIG. 60 is a flow chart illustrating another example of generation of aprediction image;

FIG. 61 is a flow chart illustrating one example of a correction processof a prediction image by overlapped block motion compensation (OBMC);

FIG. 62 is a conceptual diagram for illustrating one example of aprediction image correction process by OBMC;

FIG. 63 is a diagram for illustrating a model assuming uniform linearmotion;

FIG. 64 is a flow chart illustrating one example of inter predictionaccording to BIO;

FIG. 65 is a diagram illustrating one example of a configuration of aninter predictor which performs inter prediction according to BIO;

FIG. 66A is a diagram for illustrating one example of a prediction imagegeneration method using a luminance correction process by localillumination compensation (LIC);

FIG. 66B is a flow chart illustrating one example of a prediction imagegeneration method using a luminance correction process by LIC;

FIG. 67 is a block diagram illustrating a configuration of a decoderaccording to an embodiment;

FIG. 68 is a block diagram illustrating a mounting example of a decoder;

FIG. 69 is a flow chart illustrating one example of an overall decodingprocess performed by the decoder;

FIG. 70 is a diagram illustrating a relationship between a splittingdeterminer and other constituent elements;

FIG. 71 is a block diagram illustrating one example of a configurationof an entropy decoder;

FIG. 72 is a diagram illustrating a flow of CABAC in the entropydecoder;

FIG. 73 is a block diagram illustrating one example of a configurationof an inverse quantizer;

FIG. 74 is a flow chart illustrating one example of inverse quantizationperformed by the inverse quantizer;

FIG. 75 is a flow chart illustrating one example of a process performedby an inverse transformer;

FIG. 76 is a flow chart illustrating another example of a processperformed by the inverse transformer;

FIG. 77 is a block diagram illustrating one example of a configurationof a loop filter;

FIG. 78 is a flow chart illustrating one example of a process performedby a predictor of the decoder;

FIG. 79 is a flow chart illustrating another example of a processperformed by the predictor of the decoder;

FIG. 80A is a flow chart illustrating a portion of other example of aprocess performed by the predictor of the decoder;

FIG. 80B is a flow chart illustrating the remaining portion of the otherexample of the process performed by the predictor of the decoder;

FIG. 81 is a diagram illustrating one example of a process performed byan intra predictor of the decoder;

FIG. 82 is a flow chart illustrating one example of MV derivation in thedecoder;

FIG. 83 is a flow chart illustrating another example of MV derivation inthe decoder;

FIG. 84 is a flow chart illustrating an example of inter prediction bynormal inter mode in the decoder;

FIG. 85 is a flow chart illustrating an example of inter prediction bynormal merge mode in the decoder;

FIG. 86 is a flow chart illustrating an example of inter prediction byFRUC mode in the decoder;

FIG. 87 is a flow chart illustrating an example of inter prediction byaffine merge mode in the decoder;

FIG. 88 is a flow chart illustrating an example of inter prediction byaffine inter mode in the decoder;

FIG. 89 is a flow chart illustrating an example of inter prediction bytriangle mode in the decoder;

FIG. 90 is a flow chart illustrating an example of motion estimation byDMVR in the decoder;

FIG. 91 is a flow chart illustrating one specific example of motionestimation by DMVR in the decoder;

FIG. 92 is a flow chart illustrating one example of generation of aprediction image in the decoder;

FIG. 93 is a flow chart illustrating another example of generation of aprediction image in the decoder;

FIG. 94 is a flow chart illustrating another example of correction of aprediction image by OBMC in the decoder;

FIG. 95 is a flow chart illustrating another example of correction of aprediction image by BIO in the decoder;

FIG. 96 is a flow chart illustrating another example of correction of aprediction image by LIC in the decoder;

FIG. 97 is a diagram for describing an intra block copy (IBC) mode;

FIG. 98 is a flow chart illustrating one example of operations performedby an encoder and a decoder according to a first aspect;

FIG. 99 is a flow chart illustrating one example of operations performedby an encoder and a decoder according to a second aspect;

FIG. 100 is a flow chart illustrating operations performed by anencoder;

FIG. 101 is a flow chart illustrating operations performed by a decoder;

FIG. 102 is a diagram illustrating an overall configuration of a contentproviding system for implementing a content distribution service;

FIG. 103 is a diagram illustrating an example of a display screen of aweb page;

FIG. 104 is a diagram illustrating an example of a display screen of aweb page;

FIG. 105 is a diagram illustrating one example of a smartphone; and

FIG. 106 is a block diagram illustrating an example of a configurationof a smartphone.

DETAILED DESCRIPTION OF THE EMBODIMENTS Introduction

An encoder according to an aspect of the present disclosure is anencoder including: circuitry; and memory coupled to the circuitry. Thecircuitry, in operation: in generation of a prediction image of acurrent block, uses an intra block copy (IBC) mode in which a processedregion of a picture that includes the current block is referred to;determines whether a size of the current block is less than or equal toa threshold, the current block being a unit for which a vector candidatelist including vector candidates is generated; when the size of thecurrent block is less than or equal to the threshold, generates thevector candidate list by registering a history-based motion vectorpredictor (HMVP) vector candidate in the vector candidate list from anHMVP table without performing a first pruning process, the HMVP tablestoring, by a first-in first-out (FIFO) method, first vector candidateseach as the HMVP vector candidate, the first vector candidates eachhaving information regarding a first vector used for a processed block;when the size of the current block is greater than the threshold,generates the vector candidate list by performing the first pruningprocess and registering the HMVP vector candidate in the vectorcandidate list from the HMVP table; and encodes the current block usingthe vector candidate list.

Accordingly, the encoder generates the vector candidate list withoutperforming the first pruning process (hereinafter simply referred to asa pruning process) when the size of the current block is less than orequal to the threshold, and thus, the amount of processing is reduced.This enhances the coding efficiency of the encoder.

For example, the circuitry may: generate the prediction image of thecurrent block using a second vector; encode the second vector using oneof the vector candidates included in the vector candidate list; in thefirst pruning process performed in generation of the vector candidatelist, determine, for each of HMVP vector candidates each of which is theHMVP vector candidate stored in the HMVP table, whether the HMVP vectorcandidate is identical to any of one or more vector candidatesregistered in the vector candidate list; and when the HMVP vectorcandidate is not identical to any of the one or more vector candidates,generate the vector candidate list by registering the HMVP vectorcandidate in the vector candidate list.

Accordingly, the encoder generates the vector candidate list withoutperforming the pruning process when the size of the current block isless than or equal to the threshold, and thus, the amount of processingis reduced. This enhances the coding efficiency of the encoder.

For example, the circuitry may further update the HMVP table using asecond vector candidate having information regarding the second vector,and in update of the HMVP table, the circuitry may: determine whetherthe size of the current block is less than or equal to the threshold;update the HMVP table without performing a second pruning process whenthe size of the current block is less than or equal to the threshold;and update the HMVP table by performing the second pruning process whenthe size of the current block is greater than the threshold.

Accordingly, the encoder updates the HMVP table without performing thesecond pruning process (hereinafter also simply referred to as a pruningprocess) when the size of the current block is less than or equal to thethreshold, and thus, the amount of processing is reduced. This enhancesthe coding efficiency of the encoder.

For example, in the second pruning process performed in the update ofthe HMVP table, the circuitry may: determine whether the second vectorcandidate is identical to any of the HMVP vector candidates; and updatethe HMVP table by storing the second vector candidate in the HMVP tablewhen the second vector candidate is not identical to any of the HMVPvector candidates.

By doing so, the encoder can register, in the vector candidate list, avector candidate that is more appropriate for the current block.

For example, the size of the current block may be defined by a totalnumber of pixels included in the current block. The threshold may be 16pixels, for example.

Accordingly, the encoder generates the vector candidate list and updatesthe HMVP table without performing the pruning process when the totalnumber of pixels included in the current block is less than or equal toa threshold (for example, less than or equal to 16 pixels), and thus,the amount of processing is reduced. This enhances the coding efficiencyof the encoder.

For example, the size of the current block may be defined by at leastone of a width or a height of the current block. The threshold may be 4pixels×4 pixels, for example.

Accordingly, the encoder generates the vector candidate list and updatesthe HMVP table without performing the pruning process when at least oneof the width or the height of the current block is less than or equal toa threshold (for example, when both the width and the height of thecurrent block are less than or equal to 4 pixels), and thus, the amountof processing is reduced. This enhances the coding efficiency of theencoder.

For example, the vector candidate list need not be shared between thecurrent block and a block neighboring the current block.

Accordingly, the encoder uses the vector candidate lists each generatedfor a different current block, and thus, the prediction precision isenhanced.

For example, the circuitry may: in the generation of the predictionimage of the current block, use the IBC mode when determining to use theIBC mode among a plurality of prediction modes, and use a predictionmode different from the IBC mode when determining to use the predictionmode different from the IBC mode among the plurality of predictionmodes; when using the prediction mode different from the IBC mode,generate the vector candidate list by performing the first pruningprocess and registering the HMVP vector candidate in the vectorcandidate list from the HMVP table; and encode the current block usingthe vector candidate list.

This enables the encoder to appropriately switch the conditions underwhich the pruning process is performed, according to the prediction modeused. This enhances the coding efficiency of the encoder.

Also, a decoder according to an aspect of the present disclosure is adecoder including: circuitry; and memory coupled to the circuitry. Thecircuitry, in operation: in generation of a prediction image of acurrent block, uses an intra block copy (IBC) mode in which a processedregion of a picture that includes the current block is referred to;determines whether a size of the current block is less than or equal toa threshold, the current block being a unit for which a vector candidatelist including vector candidates is generated; when the size of thecurrent block is less than or equal to the threshold, generates thevector candidate list by registering a history-based motion vectorpredictor (HMVP) vector candidate in the vector candidate list from anHMVP table without performing a first pruning process, the HMVP tablestoring, by a first-in first-out (FIFO) method, first vector candidateseach as the HMVP vector candidate, the first vector candidates eachhaving information regarding a first vector used for a processed block;when the size of the current block is greater than the threshold,generates the vector candidate list by performing the first pruningprocess and registering the HMVP vector candidate in the vectorcandidate list from the HMVP table; and decodes the current block usingthe vector candidate list.

Accordingly, the decoder generates the vector candidate list withoutperforming the pruning process when the size of the current block isless than or equal to the threshold, and thus, the amount of processingis reduced. This enhances the processing efficiency of the decoder.

For example, the circuitry may: generate the prediction image of thecurrent block using a second vector; decode the second vector using oneof the vector candidates included in the vector candidate list; in thefirst pruning process performed in generation of the vector candidatelist, determine, for each of HMVP vector candidates each of which is theHMVP vector candidate stored in the HMVP table, whether the HMVP vectorcandidate is identical to any of one or more vector candidatesregistered in the vector candidate list; and when the HMVP vectorcandidate is not identical to any of the one or more vector candidates,generate the vector candidate list by registering the HMVP vectorcandidate in the vector candidate list.

Accordingly, the decoder generates the vector candidate list withoutperforming the pruning process when the size of the current block isless than or equal to the threshold, and thus, the amount of processingis reduced. This enhances the processing efficiency of the decoder.

For example, the circuitry may further: update the HMVP table using asecond vector candidate having information regarding the second vector,and in update of the HMVP table, the circuitry may: determine whetherthe size of the current block is less than or equal to the threshold;update the HMVP table without performing a second pruning process whenthe size of the current block is less than or equal to the threshold;and update the HMVP table by performing the second pruning process whenthe size of the current block is greater than the threshold.

Accordingly, the decoder updates the HMVP table without performing thepruning process when the size of the current block is less than or equalto the threshold, and thus, the amount of processing is reduced. Thisenhances the processing efficiency of the decoder.

For example, in the second pruning process performed in the update ofthe HMVP table, the circuitry may: determine whether the second vectorcandidate is identical to any of the HMVP vector candidates; and updatethe HMVP table by storing the second vector candidate in the HMVP tablewhen the second vector candidate is not identical to any of the HMVPvector candidates.

By doing so, the decoder can register, in the vector candidate list, avector candidate that is more appropriate for the current block.

For example, the size of the current block may be defined by a totalnumber of pixels included in the current block. The threshold may be 16pixels, for example.

Accordingly, the decoder generates the vector candidate list and updatesthe HMVP table without performing the pruning process when the totalnumber of pixels included in the current block is less than or equal toa threshold (for example, less than or equal to 16 pixels), and thus,the amount of processing is reduced. This enhances the processingefficiency of the decoder.

For example, the size of the current block may be defined by at leastone of a width or a height of the current block. The threshold may be 4pixels×4 pixels, for example.

Accordingly, the decoder generates the vector candidate list and updatesthe HMVP table without performing the pruning process when at least oneof the width or the height of the current block is less than or equal toa threshold (for example, when both the width and the height of thecurrent block are less than or equal to 4 pixels), and thus, the amountof processing is reduced. This enhances the processing efficiency of thedecoder.

For example, the vector candidate list need not be shared between thecurrent block and a block neighboring the current block.

Accordingly, the decoder uses the vector candidate lists each generatedfor a different current block, and thus, the prediction precision isenhanced.

For example, the circuitry may: in the generation of the predictionimage of the current block, use the IBC mode when determining to use theIBC mode among a plurality of prediction modes, and use a predictionmode different from the IBC mode when determining to use the predictionmode different from the IBC mode among the plurality of predictionmodes; when using the prediction mode different from the IBC mode,generate the vector candidate list by performing the first pruningprocess and registering the HMVP vector candidate in the vectorcandidate list from the HMVP table; and decode the current block usingthe vector candidate list.

This enables the decoder to appropriately switch conditions under whichthe pruning process is performed, according to the prediction mode used.This enhances the processing efficiency of the decoder.

Also, an encoding method according to an aspect of the presentdisclosure is an encoding method including: in generation of aprediction image of a current block, using an intra block copy (IBC)mode in which a processed region of a picture that includes the currentblock is referred to; determining whether a size of the current block isless than or equal to a threshold, the current block being a unit forwhich a vector candidate list including vector candidates is generated;when the size of the current block is less than or equal to thethreshold, generating the vector candidate list by registering ahistory-based motion vector predictor (HMVP) vector candidate in thevector candidate list from an HMVP table without performing a firstpruning process, the HMVP table storing, by a first-in first-out (FIFO)method, first vector candidates each as the HMVP vector candidate, thefirst vector candidates each having information regarding a first vectorused for a processed block; when the size of the current block isgreater than the threshold, generating the vector candidate list byperforming the first pruning process and registering the HMVP vectorcandidate in the vector candidate list from the HMVP table; and encodingthe current block using the vector candidate list.

Accordingly, a device that executes the encoding method generates thevector candidate list without performing the pruning process when thesize of the current block is less than or equal to the threshold, andthus, the amount of processing is reduced. This enhances the codingefficiency of the device that executes the encoding method.

Also, a decoding method according to an aspect of the present disclosureis a decoding method including: in generation of a prediction image of acurrent block, using an intra block copy (IBC) mode in which a processedregion of a picture that includes the current block is referred to;determining whether a size of the current block is less than or equal toa threshold, the current block being a unit for which a vector candidatelist including vector candidates is generated; when the size of thecurrent block is less than or equal to the threshold, generating thevector candidate list by registering a history-based motion vectorpredictor (HMVP) vector candidate in the vector candidate list from anHMVP table without performing a first pruning process, the HMVP tablestoring, by a first-in first-out (FIFO) method, first vector candidateseach as the HMVP vector candidate, the first vector candidates eachhaving information regarding a first vector used for a processed block;when the size of the current block is greater than the threshold,generating the vector candidate list by performing the first pruningprocess and registering the HMVP vector candidate in the vectorcandidate list from the HMVP table; and decoding the current block usingthe vector candidate list.

Accordingly, a device that executes the decoding method generates thevector candidate list without performing the pruning process when thesize of the current block is less than or equal to the threshold, andthus, the amount of processing is reduced. This enhances the processingefficiency of the device that executes the decoding method.

Definitions of Terms

The respective terms may be defined as indicated below as examples.

(1) Image

An image is a data unit configured with a set of pixels, is a picture orincludes blocks smaller than a picture. Images include a still image inaddition to a video.

(2) Picture

A picture is an image processing unit configured with a set of pixels,and is also referred to as a frame or a field.

(3) Block

A block is a processing unit which is a set of a particular number ofpixels. The block is also referred to as indicated in the followingexamples. The shapes of blocks are not limited. Examples include arectangle shape of M×N pixels and a square shape of M×M pixels for thefirst place, and also include a triangular shape, a circular shape, andother shapes.

Examples of Blocks

-   -   slice/tile/brick    -   CTU/super block/basic splitting unit    -   VPDU/processing splitting unit for hardware    -   CU/processing block unit/prediction block unit (PU)/orthogonal        transform block unit (TU)/unit    -   sub-block

(4) Pixel/Sample

A pixel or sample is a smallest point of an image. Pixels or samplesinclude not only a pixel at an integer position but also a pixel at asub-pixel position generated based on a pixel at an integer position.

(5) Pixel Value/Sample Value

A pixel value or sample value is an eigen value of a pixel. Pixel orsample values naturally include a luma value, a chroma value, an RGBgradation level and also covers a depth value, or a binary value of 0 or1.

(6) Flag

A flag indicates one or more bits, and may be, for example, a parameteror index represented by two or more bits. Alternatively, the flag mayindicate not only a binary value represented by a binary number but alsoa multiple value represented by a number other than the binary number.

(7) Signal

A signal is the one symbolized or encoded to convey information. Signalsinclude a discrete digital signal and an analog signal which takes acontinuous value.

(8) Stream/Bitstream

A stream or bitstream is a digital data string or a digital data flow. Astream or bitstream may be one stream or may be configured with aplurality of streams having a plurality of hierarchical layers. A streamor bitstream may be transmitted in serial communication using a singletransmission path, or may be transmitted in packet communication using aplurality of transmission paths.

(9) Difference

In the case of scalar quantity, it is only necessary that a simpledifference (x− y) and a difference calculation be included. Differencesinclude an absolute value of a difference (|x−y|), a squared difference(x′− y′), a square root of a difference (M(x− y)), a weighted difference(ax− by: a and b are constants), an offset difference (x− y+a: a is anoffset).

(10) Sum

In the case of scalar quantity, it is only necessary that a simple sum(x+y) and a sum calculation be included. Sums include an absolute valueof a sum (|x+y|), a squared sum (x′+y′), a square root of a sum(<(x+y)), a weighted difference (ax+by: a and b are constants), anoffset sum (x+y+a: a is an offset).

(11) Based on

A phrase “based on something” means that a thing other than thesomething may be considered. In addition, “based on” may be used in acase in which a direct result is obtained or a case in which a result isobtained through an intermediate result.

(12) Used, Using

A phrase “something used” or “using something” means that a thing otherthan the something may be considered. In addition, “used” or “using” maybe used in a case in which a direct result is obtained or a case inwhich a result is obtained through an intermediate result.

(13) Prohibit, Forbid

The term “prohibit” or “forbid” can be rephrased as “does not permit” or“does not allow”. In addition, “being not prohibited/forbidden” or“being permitted/allowed” does not always mean “obligation”.

(14) Limit, Restriction/Restrict/Restricted

The term “limit” or “restriction/restrict/restricted” can be rephrasedas “does not permit/allow” or “being not permitted/allowed”. Inaddition, “being not prohibited/forbidden” or “being permitted/allowed”does not always mean “obligation”. Furthermore, it is only necessarythat part of something be prohibited/forbidden quantitatively orqualitatively, and something may be fully prohibited/forbidden.

(15) Chroma

An adjective, represented by the symbols Cb and Cr, specifying that asample array or single sample is representing one of the two colordifference signals related to the primary colors. The term chroma may beused instead of the term chrominance.

(16) Luma

An adjective, represented by the symbol or subscript Y or L, specifyingthat a sample array or single sample is representing the monochromesignal related to the primary colors. The term luma may be used insteadof the term luminance.

[Notes Related to the Descriptions]

In the drawings, same reference numbers indicate same or similarcomponents. The sizes and relative locations of components are notnecessarily drawn by the same scale.

Hereinafter, embodiments will be described with reference to thedrawings. Note that the embodiments described below each show a generalor specific example. The numerical values, shapes, materials,components, the arrangement and connection of the components, steps, therelation and order of the steps, etc., indicated in the followingembodiments are mere examples, and are not intended to limit the scopeof the claims.

Embodiments of an encoder and a decoder will be described below. Theembodiments are examples of an encoder and a decoder to which theprocesses and/or configurations presented in the description of aspectsof the present disclosure are applicable. The processes and/orconfigurations can also be implemented in an encoder and a decoderdifferent from those according to the embodiments. For example,regarding the processes and/or configurations as applied to theembodiments, any of the following may be implemented:

(1) Any of the components of the encoder or the decoder according to theembodiments presented in the description of aspects of the presentdisclosure may be substituted or combined with another componentpresented anywhere in the description of aspects of the presentdisclosure.

(2) In the encoder or the decoder according to the embodiments,discretionary changes may be made to functions or processes performed byone or more components of the encoder or the decoder, such as addition,substitution, removal, etc., of the functions or processes. For example,any function or process may be substituted or combined with anotherfunction or process presented anywhere in the description of aspects ofthe present disclosure.

(3) In methods implemented by the encoder or the decoder according tothe embodiments, discretionary changes may be made such as addition,substitution, and removal of one or more of the processes included inthe method. For example, any process in the method may be substituted orcombined with another process presented anywhere in the description ofaspects of the present disclosure.

(4) One or more components included in the encoder or the decoderaccording to embodiments may be combined with a component presentedanywhere in the description of aspects of the present disclosure, may becombined with a component including one or more functions presentedanywhere in the description of aspects of the present disclosure, andmay be combined with a component that implements one or more processesimplemented by a component presented in the description of aspects ofthe present disclosure.

(5) A component including one or more functions of the encoder or thedecoder according to the embodiments, or a component that implements oneor more processes of the encoder or the decoder according to theembodiments, may be combined or substituted with a component presentedanywhere in the description of aspects of the present disclosure, with acomponent including one or more functions presented anywhere in thedescription of aspects of the present disclosure, or with a componentthat implements one or more processes presented anywhere in thedescription of aspects of the present disclosure.

(6) In methods implemented by the encoder or the decoder according tothe embodiments, any of the processes included in the method may besubstituted or combined with a process presented anywhere in thedescription of aspects of the present disclosure or with anycorresponding or equivalent process.

(7) One or more processes included in methods implemented by the encoderor the decoder according to the embodiments may be combined with aprocess presented anywhere in the description of aspects of the presentdisclosure.

(8) The implementation of the processes and/or configurations presentedin the description of aspects of the present disclosure is not limitedto the encoder or the decoder according to the embodiments. For example,the processes and/or configurations may be implemented in a device usedfor a purpose different from the moving picture encoder or the movingpicture decoder disclosed in the embodiments.

[System Configuration]

FIG. 1 is a schematic diagram illustrating one example of aconfiguration of a transmission system according to an embodiment.

Transmission system Trs is a system which transmits a stream generatedby encoding an image and decodes the transmitted stream. Transmissionsystem Trs like this includes, for example, encoder 100, network Nw, anddecoder 200 as illustrated in FIG. 1 .

An image is input to encoder 100. Encoder 100 generates a stream byencoding the input image, and outputs the stream to network Nw. Thestream includes, for example, the encoded image and control informationfor decoding the encoded image. The image is compressed by the encoding.

It is to be noted that a previous image before being encoded and beinginput to encoder 100 is also referred to as the original image, theoriginal signal, or the original sample. The image may be a video or astill image. The image is a generic concept of a sequence, a picture,and a block, and thus is not limited to a spatial region having aparticular size and to a temporal region having a particular size unlessotherwise specified. The image is an array of pixels or pixel values,and the signal representing the image or pixel values are also referredto as samples. The stream may be referred to as a bitstream, an encodedbitstream, a compressed bitstream, or an encoded signal. Furthermore,the encoder may be referred to as an image encoder or a video encoder.The encoding method performed by encoder 100 may be referred to as anencoding method, an image encoding method, or a video encoding method.

Network Nw transmits the stream generated by encoder 100 to decoder 200.Network Nw may be the Internet, the Wide Area Network (WAN), the LocalArea Network (LAN), or any combination of these networks. Network Nw isnot always limited to a bi-directional communication network, and may bea uni-directional communication network which transmits broadcast wavesof digital terrestrial broadcasting, satellite broadcasting, or thelike. Alternatively, network Nw may be replaced by a medium such as aDigital Versatile Disc (DVD) and a Blu-Ray Disc (BD)®, etc. on which astream is recorded.

Decoder 200 generates, for example, a decoded image which is anuncompressed image by decoding a stream transmitted by network Nw. Forexample, the decoder decodes a stream according to a decoding methodcorresponding to an encoding method by encoder 100.

It is to be noted that the decoder may also be referred to as an imagedecoder or a video decoder, and that the decoding method performed bydecoder 200 may also be referred to as a decoding method, an imagedecoding method, or a video decoding method.

[Data Structure]

FIG. 2 is a diagram illustrating one example of a hierarchical structureof data in a stream. A stream includes, for example, a video sequence.As illustrated in (a) of FIG. 2 , the video sequence includes a videoparameter set (VPS), a sequence parameter set (SPS), a picture parameterset (PPS), supplemental enhancement information (SEI), and a pluralityof pictures.

In a video having a plurality of layers, a VPS includes: a codingparameter which is common between some of the plurality of layers; and acoding parameter related to some of the plurality of layers included inthe video or an individual layer.

An SPS includes a parameter which is used for a sequence, that is, acoding parameter which decoder 200 refers to in order to decode thesequence. For example, the coding parameter may indicate the width orheight of a picture. It is to be noted that a plurality of SPSs may bepresent.

A PPS includes a parameter which is used for a picture, that is, acoding parameter which decoder 200 refers to in order to decode each ofthe pictures in the sequence. For example, the coding parameter mayinclude a reference value for the quantization width which is used todecode a picture and a flag indicating application of weightedprediction. It is to be noted that a plurality of PPSs may be present.Each of the SPS and the PPS may be simply referred to as a parameterset.

As illustrated in (b) of FIG. 2 , a picture may include a picture headerand at least one slice. A picture header includes a coding parameterwhich decoder 200 refers to in order to decode the at least one slice.

As illustrated in (c) of FIG. 2 , a slice includes a slice header and atleast one brick. A slice header includes a coding parameter whichdecoder 200 refers to in order to decode the at least one brick.

As illustrated in (d) of FIG. 2 , a brick includes at least one codingtree unit (CTU).

It is to be noted that a picture may not include any slice and mayinclude a tile group instead of a slice. In this case, the tile groupincludes at least one tile. In addition, a brick may include a slice.

A CTU is also referred to as a super block or a basis splitting unit. Asillustrated in (e) of FIG. 2 , a CTU like this includes a CTU header andat least one coding unit (CU). A CTU header includes a coding parameterwhich decoder 200 refers to in order to decode the at least one CU.

A CU may be split into a plurality of smaller CUs. As illustrated in (f)of FIG. 2 , a CU includes a CU header, prediction information, andresidual coefficient information. Prediction information is informationfor predicting the CU, and the residual coefficient information isinformation indicating a prediction residual to be described later.Although a CU is basically the same as a prediction unit (PU) and atransform unit (TU), it is to be noted that, for example, an SBT to bedescribed later may include a plurality of TUs smaller than the CU. Inaddition, the CU may be processed for each virtual pipeline decodingunit (VPDU) included in the CU. The VPDU is, for example, a fixed unitwhich can be processed at one stage when pipeline processing isperformed in hardware.

It is to be noted that a stream may not include part of the hierarchicallayers illustrated in FIG. 2 . The order of the hierarchical layers maybe exchanged, or any of the hierarchical layers may be replaced byanother hierarchical layer. Here, a picture which is a target for aprocess which is about to be performed by a device such as encoder 100or decoder 200 is referred to as a current picture. A current picturemeans a current picture to be encoded when the process is an encodingprocess, and a current picture means a current picture to be decodedwhen the process is a decoding process. Likewise, for example, a CU or ablock of CUs which is a target for a process which is about to beperformed by a device such as encoder 100 or decoder 200 is referred toas a current block. A current block means a current block to be encodedwhen the process is an encoding process, and a current block means acurrent block to be decoded when the process is a decoding process.

[Picture Structure: Slice/Tile]

A picture may be configured with one or more slice units or tile unitsin order to decode the picture in parallel.

Slices are basic encoding units included in a picture. A picture mayinclude, for example, one or more slices. In addition, a slice includesone or more successive coding tree units (CTUs).

FIG. 3 is a diagram illustrating one example of a slice configuration.For example, a picture includes 11×8 CTUs, and is split into four slices(slices 1 to 4). Slice 1 includes sixteen CTUs, slice 2 includestwenty-one CTUs, slice 3 includes twenty-nine CTUs, and slice 4 includestwenty-two CTUs. Here, each CTU in the picture belongs to one of theslices. The shape of each slice is a shape obtained by splitting thepicture horizontally. A boundary of each slice does not need to coincidewith an image end, and may coincide with any of the boundaries betweenCTUs in the image. The processing order of the CTUs in a slice (anencoding order or a decoding order) is, for example, a raster-scanorder. A slice includes a slice header and encoded data. Features of theslice may be written in the slice header. The features include a CTUaddress of a top CTU in the slice, a slice type, etc.

A tile is a unit of a rectangular region included in a picture. Each oftiles may be assigned with a number referred to as TileId in raster-scanorder.

FIG. 4 is a diagram illustrating one example of a tile configuration.For example, a picture includes 11×8 CTUs, and is split into four tilesof rectangular regions (tiles 1 to 4). When tiles are used, theprocessing order of CTUs is changed from the processing order in thecase where no tile is used. When no tile is used, a plurality of CTUs ina picture are processed in raster-scan order. When a plurality of tilesare used, at least one CTU in each of the plurality of tiles isprocessed in raster-scan order. For example, as illustrated in FIG. 4 ,the processing order of the CTUs included in tile 1 is the order whichstarts from the left-end of the first column of tile 1 toward theright-end of the first column of tile 1 and then starts from theleft-end of the second column of tile 1 toward the right-end of thesecond column of tile 1.

It is to be noted that one tile may include one or more slices, and oneslice may include one or more tiles.

It is to be noted that a picture may be configured with one or more tilesets. A tile set may include one or more tile groups, or one or moretiles. A picture may be configured with only one of a tile set, a tilegroup, and a tile. For example, an order for scanning a plurality oftiles for each tile set in raster scan order is assumed to be a basicencoding order of tiles. A set of one or more tiles which are continuousin the basic encoding order in each tile set is assumed to be a tilegroup. Such a picture may be configured by splitter 102 (see FIG. 7 ) tobe described later.

[Scalable Encoding]

FIGS. 5 and 6 are diagrams illustrating examples of scalable streamstructures.

As illustrated in FIG. 5 , encoder 100 may generate atemporally/spatially scalable stream by dividing each of a plurality ofpictures into any of a plurality of layers and encoding the picture inthe layer. For example, encoder 100 encodes the picture for each layer,thereby achieving scalability where an enhancement layer is presentabove a base layer. Such encoding of each picture is also referred to asscalable encoding. In this way, decoder 200 is capable of switchingimage quality of an image which is displayed by decoding the stream. Inother words, decoder 200 determines up to which layer to decode based oninternal factors such as the processing ability of decoder 200 andexternal factors such as a state of a communication bandwidth. As aresult, decoder 200 is capable of decoding a content while freelyswitching between low resolution and high resolution. For example, theuser of the stream watches a video of the stream halfway using asmartphone on the way to home, and continues watching the video at homeon a device such as a TV connected to the Internet. It is to be notedthat each of the smartphone and the device described above includesdecoder 200 having the same or different performances. In this case,when the device decodes layers up to the higher layer in the stream, theuser can watch the video at high quality at home. In this way, encoder100 does not need to generate a plurality of streams having differentimage qualities of the same content, and thus the processing load can bereduced.

Furthermore, the enhancement layer may include meta information based onstatistical information on the image. Decoder 200 may generate a videowhose image quality has been enhanced by performing super-resolutionimaging on a picture in the base layer based on the metadata.Super-resolution imaging may be any of improvement in theSignal-to-Noise (SN) ratio in the same resolution and increase inresolution. Metadata may include information for identifying a linear ora non-linear filter coefficient, as used in a super-resolution process,or information identifying a parameter value in a filter process,machine learning, or a least squares method used in super-resolutionprocessing.

Alternatively, a configuration may be provided in which a picture isdivided into, for example, tiles in accordance with, for example, themeaning of an object in the picture. In this case, decoder 200 maydecode only a partial region in a picture by selecting a tile to bedecoded. In addition, an attribute of the object (person, car, ball,etc.) and a position of the object in the picture (coordinates inidentical images) may be stored as metadata. In this case, decoder 200is capable of identifying the position of a desired object based on themetadata, and determining the tile including the object. For example, asillustrated in FIG. 6 , the metadata may be stored using a data storagestructure different from image data, such as SEI in HEVC. This metadataindicates, for example, the position, size, or color of a main object.

Metadata may be stored in units of a plurality of pictures, such as astream, a sequence, or a random access unit. In this way, decoder 200 iscapable of obtaining, for example, the time at which a specific personappears in the video, and by fitting the time information with pictureunit information, is capable of identifying a picture in which theobject is present and determining the position of the object in thepicture.

[Encoder]

Next, encoder 100 according to this embodiment is described. FIG. 7 is ablock diagram illustrating one example of a configuration of encoder 100according to this embodiment. Encoder 100 encodes an image in units of ablock.

As illustrated in FIG. 7 , encoder 100 is an apparatus which encodes animage in units of a block, and includes splitter 102, subtractor 104,transformer 106, quantizer 108, entropy encoder 110, inverse quantizer112, inverse transformer 114, adder 116, block memory 118, loop filter120, frame memory 122, intra predictor 124, inter predictor 126,prediction controller 128, and prediction parameter generator 130. It isto be noted that intra predictor 124 and inter predictor 126 areconfigured as part of a prediction executor.

[Mounting Example of Encoder]

FIG. 8 is a block diagram illustrating a mounting example of encoder100. Encoder 100 includes processor a1 and memory a2. For example, theplurality of constituent elements of encoder 100 illustrated in FIG. 7are mounted on processor a1 and memory a2 illustrated in FIG. 8 .

Processor a1 is circuitry which performs information processing and isaccessible to memory a2. For example, processor a1 is dedicated orgeneral electronic circuitry which encodes an image. Processor a1 may bea processor such as a CPU. In addition, processor a1 may be an aggregateof a plurality of electronic circuits. In addition, for example,processor a1 may take the roles of two or more constituent elementsother than a constituent element for storing information out of theplurality of constituent elements of encoder 100 illustrated in FIG. 7 ,etc.

Memory a2 is dedicated or general memory for storing information that isused by processor a1 to encode the image. Memory a2 may be electroniccircuitry, and may be connected to processor a1. In addition, memory a2may be included in processor a1. In addition, memory a2 may be anaggregate of a plurality of electronic circuits. In addition, memory a2may be a magnetic disc, an optical disc, or the like, or may berepresented as storage, a medium, or the like. In addition, memory a2may be non-volatile memory, or volatile memory.

For example, memory a2 may store an image to be encoded or a streamcorresponding to an encoded image. In addition, memory a2 may store aprogram for causing processor a1 to encode an image.

In addition, for example, memory a2 may take the roles of two or moreconstituent elements for storing information out of the plurality ofconstituent elements of encoder 100 illustrated in FIG. 7 . Morespecifically, memory a2 may take the roles of block memory 118 and framememory 122 illustrated in FIG. 7 . More specifically, memory a2 maystore a reconstructed image (specifically, a reconstructed block, areconstructed picture, or the like).

It is to be noted that, in encoder 100, not all of the plurality ofconstituent elements indicated in FIG. 7 , etc. may be implemented, andnot all the processes described above may be performed. Part of theconstituent elements indicated in FIG. 7 may be included in anotherdevice, or part of the processes described above may be performed byanother device.

Hereinafter, an overall flow of processes performed by encoder 100 isdescribed, and then each of constituent elements included in encoder 100is described.

[Overall Flow of Encoding Process]

FIG. 9 is a flow chart illustrating one example of an overall encodingprocess performed by encoder 100.

First, splitter 102 of encoder 100 splits each of pictures included inan original image into a plurality of blocks having a fixed size(128×128 pixels) (Step Sa_1). Splitter 102 then selects a splittingpattern for the fixed-size block (Step Sa_2). In other words, splitter102 further splits the fixed-size block into a plurality of blocks whichform the selected splitting pattern. Encoder 100 performs, for each ofthe plurality of blocks, Steps Sa_3 to Sa_9 for the block.

Prediction controller 128 and a prediction executor which is configuredwith intra predictor 124 and inter predictor 126 generate a predictionimage of a current block (Step Sa_3). It is to be noted that theprediction image is also referred to as a prediction signal, aprediction block, or prediction samples.

Next, subtractor 104 generates the difference between a current blockand a prediction image as a prediction residual (Step Sa_4). It is to benoted that the prediction residual is also referred to as a predictionerror.

Next, transformer 106 transforms the prediction image and quantizer 108quantizes the result, to generate a plurality of quantized coefficients(Step Sa_5).

Next, entropy encoder 110 encodes (specifically, entropy encodes) theplurality of quantized coefficients and a prediction parameter relatedto generation of a prediction image to generate a stream (Step Sa_6).

Next, inverse quantizer 112 performs inverse quantization of theplurality of quantized coefficients and inverse transformer 114 performsinverse transform of the result, to restore a prediction residual (StepSa_7).

Next, adder 116 adds the prediction image to the restored predictionresidual to reconstruct the current block (Step Sa_8). In this way, thereconstructed image is generated. It is to be noted that thereconstructed image is also referred to as a reconstructed block, and,in particular, that a reconstructed image generated by encoder 100 isalso referred to as a local decoded block or a local decoded image.

When the reconstructed image is generated, loop filter 120 performsfiltering of the reconstructed image as necessary (Step Sa_9).

Encoder 100 then determines whether encoding of the entire picture hasbeen finished (Step Sa_10). When determining that the encoding has notyet been finished (No in Step Sa_10), processes from Step Sa_2 areexecuted repeatedly.

Although encoder 100 selects one splitting pattern for a fixed-sizeblock, and encodes each block according to the splitting pattern in theabove-described example, it is to be noted that each block may beencoded according to a corresponding one of a plurality of splittingpatterns. In this case, encoder 100 may evaluate a cost for each of theplurality of splitting patterns, and, for example, may select the streamobtained by encoding according to the splitting pattern which yields thesmallest cost as a stream which is output finally.

Alternatively, the processes in Steps Sa_1 to Sa_10 may be performedsequentially by encoder 100, or two or more of the processes may beperformed in parallel or may be reordered.

The encoding process by encoder 100 is hybrid encoding using predictionencoding and transform encoding. In addition, prediction encoding isperformed by an encoding loop configured with subtractor 104,transformer 106, quantizer 108, inverse quantizer 112, inversetransformer 114, adder 116, loop filter 120, block memory 118, framememory 122, intra predictor 124, inter predictor 126, and predictioncontroller 128. In other words, the prediction executor configured withintra predictor 124 and inter predictor 126 is part of the encodingloop.

[Splitter]

Splitter 102 splits each of pictures included in the original image intoa plurality of blocks, and outputs each block to subtractor 104. Forexample, splitter 102 first splits a picture into blocks of a fixed size(for example, 128×128 pixels). The fixed-size block is also referred toas a coding tree unit (CTU). Splitter 102 then splits each fixed-sizeblock into blocks of variable sizes (for example, 64×64 pixels orsmaller), based on recursive quadtree and/or binary tree blocksplitting. In other words, splitter 102 selects a splitting pattern. Thevariable-size block is also referred to as a coding unit (CU), aprediction unit (PU), or a transform unit (TU). It is to be noted that,in various kinds of mounting examples, there is no need to differentiatebetween CU, PU, and TU; all or some of the blocks in a picture may beprocessed in units of a CU, a PU, or a TU.

FIG. 10 is a diagram illustrating one example of block splittingaccording to this embodiment. In FIG. 10 , the solid lines representblock boundaries of blocks split by quadtree block splitting, and thedashed lines represent block boundaries of blocks split by binary treeblock splitting.

Here, block 10 is a square block having 128×128 pixels. This block 10 isfirst split into four square 64×64 pixel blocks (quadtree blocksplitting).

The upper-left 64×64 pixel block is further vertically split into tworectangle 32×64 pixel blocks, and the left 32×64 pixel block is furthervertically split into two rectangle 16×64 pixel blocks (binary treeblock splitting). As a result, the upper-left square 64×64 pixel blockis split into two 16×64 pixel blocks 11 and 12 and one 32×64 pixel block13.

The upper-right square 64×64 pixel block is horizontally split into tworectangle 64×32 pixel blocks 14 and 15 (binary tree block splitting).

The lower-left square 64×64 pixel block is first split into four square32×32 pixel blocks (quadtree block splitting). The upper-left block andthe lower-right block among the four square 32×32 pixel blocks arefurther split. The upper-left square 32×32 pixel block is verticallysplit into two rectangle 16×32 pixel blocks, and the right 16×32 pixelblock is further horizontally split into two 16×16 pixel blocks (binarytree block splitting). The lower-right 32×32 pixel block is horizontallysplit into two 32×16 pixel blocks (binary tree block splitting). Theupper-right square 32×32 pixel block is horizontally split into tworectangle 32×16 pixel blocks (binary tree block splitting). As a result,the lower-left square 64×64 pixel block is split into rectangle 16×32pixel block 16, two square 16×16 pixel blocks 17 and 18, two square32×32 pixel blocks 19 and 20, and two rectangle 32×16 pixel blocks 21and 22.

The lower-right 64×64 pixel block 23 is not split.

As described above, in FIG. 10 , block 10 is split into thirteenvariable-size blocks 11 through 23 based on recursive quadtree andbinary tree block splitting. Such splitting is also referred to asquad-tree plus binary tree splitting (QTBT).

It is to be noted that, in FIG. 10 , one block is split into four or twoblocks (quadtree or binary tree block splitting), but splitting is notlimited to these examples. For example, one block may be split intothree blocks (ternary block splitting). Splitting including such ternaryblock splitting is also referred to as multi type tree (MBT) splitting.

FIG. 11 is a diagram illustrating one example of a configuration ofsplitter 102. As illustrated in FIG. 11 , splitter 102 may include blocksplitting determiner 102 a. Block splitting determiner 102 a may performthe following processes as examples.

For example, block splitting determiner 102 a collects block informationfrom either block memory 118 or frame memory 122, and determines theabove-described splitting pattern based on the block information.Splitter 102 splits the original image according to the splittingpattern, and outputs at least one block obtained by the splitting tosubtractor 104.

In addition, for example, block splitting determiner 102 a outputs aparameter indicating the above-described splitting pattern totransformer 106, inverse transformer 114, intra predictor 124, interpredictor 126, and entropy encoder 110. Transformer 106 may transform aprediction residual based on the parameter. Intra predictor 124 andinter predictor 126 may generate a prediction image based on theparameter. In addition, entropy encoder 110 may entropy encodes theparameter.

The parameter related to the splitting pattern may be written in astream as indicated below as one example.

FIG. 12 is a diagram illustrating examples of splitting patterns.Examples of splitting patterns include: splitting into four regions (QT)in which a block is split into two regions both horizontally andvertically; splitting into three regions (HT or VT) in which a block issplit in the same direction in a ratio of 1:2:1; splitting into tworegions (HB or VB) in which a block is split in the same direction in aratio of 1:1; and no splitting (NS).

It is to be noted that the splitting pattern does not have any blocksplitting direction in the case of splitting into four regions and nosplitting, and that the splitting pattern has splitting directioninformation in the case of splitting into two regions or three regions.

FIGS. 13A and 13B are each a diagram illustrating one example of asyntax tree of a splitting pattern. In the example of FIG. 13A, first,information indicating whether to perform splitting (S: Split flag) ispresent, and information indicating whether to perform splitting intofour regions (QT: QT flag) is present next. Information indicating whichone of splitting into three regions and two regions is to be performed(TT: TT flag or BT: BT flag) is present next, and lastly, informationindicating a division direction (Ver: Vertical flag or Hor: Horizontalflag) is present. It is to be noted that each of at least one blockobtained by splitting according to such a splitting pattern may befurther split repeatedly in a similar process. In other words, as oneexample, whether splitting is performed, whether splitting into fourregions is performed, which one of the horizontal direction and thevertical direction is the direction in which a splitting method is to beperformed, which one of splitting into three regions and splitting intotwo regions is to be performed may be recursively determined, and thedetermination results may be encoded in a stream according to theencoding order disclosed by the syntax tree illustrated in FIG. 13A.

In addition, although information items respectively indicating S, QT,TT, and Ver are arranged in the listed order in the syntax treeillustrated in FIG. 13A, information items respectively indicating S,QT, Ver, and BT may be arranged in the listed order. In other words, inthe example of FIG. 13B, first, information indicating whether toperform splitting (S: Split flag) is present, and information indicatingwhether to perform splitting into four regions (QT: QT flag) is presentnext. Information indicating the splitting direction (Ver: Vertical flagor Hor: Horizontal flag) is present next, and lastly, informationindicating which one of splitting into two regions and splitting intothree regions is to be performed (BT: BT flag or TT: TT flag) ispresent.

It is to be noted that the splitting patterns described above areexamples, and splitting patterns other than the described splittingpatterns may be used, or part of the described splitting patterns may beused.

[Subtractor]

Subtractor 104 subtracts a prediction image (prediction image that isinput from prediction controller 128) from the original image in unitsof a block input from splitter 102 and split by splitter 102. In otherwords, subtractor 104 calculates prediction residuals of a currentblock. Subtractor 104 then outputs the calculated prediction residualsto transformer 106.

The original signal is an input signal which has been input to encoder100 and represents an image of each picture included in a video (forexample, a luma signal and two chroma signals).

[Transformer]

Transformer 106 transforms prediction residuals in spatial domain intotransform coefficients in frequency domain, and outputs the transformcoefficients to quantizer 108. More specifically, transformer 106applies, for example, a predefined discrete cosine transform (DCT) ordiscrete sine transform (DST) to prediction residuals in spatial domain.

It is to be noted that transformer 106 may adaptively select a transformtype from among a plurality of transform types, and transform predictionresiduals into transform coefficients by using a transform basisfunction corresponding to the selected transform type. This sort oftransform is also referred to as explicit multiple core transform (EMT)or adaptive multiple transform (AMT). In addition, a transform basisfunction is also simply referred to as a basis.

The transform types include, for example, DCT-II, DCT-V, DCT-VIII,DST-I, and DST-VII. It is to be noted that these transform types may berepresented as DCT2, DCT5, DCT8, DST1, and DST7. FIG. 14 is a chartillustrating transform basis functions for each transform type. In FIG.14 , N indicates the number of input pixels. For example, selection of atransform type from among the plurality of transform types may depend ona prediction type (one of intra prediction and inter prediction), andmay depend on an intra prediction mode.

Information indicating whether to apply such EMT or AMT (referred to as,for example, an EMT flag or an AMT flag) and information indicating theselected transform type is normally signaled at the CU level. It is tobe noted that the signaling of such information does not necessarilyneed to be performed at the CU level, and may be performed at anotherlevel (for example, at the sequence level, picture level, slice level,brick level, or CTU level).

In addition, transformer 106 may re-transform the transform coefficients(which are transform results). Such re-transform is also referred to asadaptive secondary transform (AST) or non-separable secondary transform(NSST). For example, transformer 106 performs re-transform in units of asub-block (for example, 4×4 pixel sub-block) included in a transformcoefficient block corresponding to an intra prediction residual.Information indicating whether to apply NSST and information related toa transform matrix for use in NSST are normally signaled at the CUlevel. It is to be noted that the signaling of such information does notnecessarily need to be performed at the CU level, and may be performedat another level (for example, at the sequence level, picture level,slice level, brick level, or CTU level).

Transformer 106 may employ a separable transform and a non-separabletransform. A separable transform is a method in which a transform isperformed a plurality of times by separately performing a transform foreach of directions according to the number of dimensions of inputs. Anon-separable transform is a method of performing a collective transformin which two or more dimensions in multidimensional inputs arecollectively regarded as a single dimension.

In one example of the non-separable transform, when an input is a 4×4pixel block, the 4×4 pixel block is regarded as a single array includingsixteen elements, and the transform applies a 16×16 transform matrix tothe array.

In another example of the non-separable transform, an input block of 4×4pixels is regarded as a single array including sixteen elements, andthen a transform (hypercube givens transform) in which givens revolutionis performed on the array a plurality of times may be performed.

In the transform in transformer 106, the transform types of transformbasis functions to be transformed into the frequency domain according toregions in a CU can be switched. Examples include a spatially varyingtransform (SVT).

FIG. 15 is a diagram illustrating one example of SVT.

In SVT, as illustrated in FIG. 15 , CUs are split into two equal regionshorizontally or vertically, and only one of the regions is transformedinto the frequency domain. A transform type can be set for each region.For example, DST7 and DST8 are used. For example, among the two regionsobtained by splitting a CU vertically into two equal regions, DST7 andDCT8 may be used for the region at position 0. Alternatively, among thetwo regions, DST7 is used for the region at position 1. Likewise, amongthe two regions obtained by splitting a CU horizontally into two equalregions, DST7 and DCT8 are used for the region at position 0.Alternatively, among the two regions, DST7 is used for the region atposition 1. Although only one of the two regions in a CU is transformedand the other is not transformed in the example illustrated in FIG. 15 ,each of the two regions may be transformed. In addition, splittingmethod may include not only splitting into two regions but alsosplitting into four regions. In addition, the splitting method can bemore flexible. For example, information indicating the splitting methodmay be encoded and may be signaled in the same manner as the CUsplitting. It is to be noted that SVT is also referred to as sub-blocktransform (SBT).

The AMT and EMT described above may be referred to as MTS (multipletransform selection). When MTS is applied, a transform type that isDST7, DCT8, or the like can be selected, and the information indicatingthe selected transform type may be encoded as index information for eachCU. There is another process referred to as IMTS (implicit MTS) as aprocess for selecting, based on the shape of a CU, a transform type tobe used for orthogonal transform performed without encoding indexinformation. When IMTS is applied, for example, when a CU has arectangle shape, orthogonal transform of the rectangle shape isperformed using DST7 for the short side and DST2 for the long side. Inaddition, for example, when a CU has a square shape, orthogonaltransform of the rectangle shape is performed using DCT2 when MTS iseffective in a sequence and using DST7 when MTS is ineffective in thesequence. DCT2 and DST7 are mere examples. Other transform types may beused, and it is also possible to change the combination of transformtypes for use to a different combination of transform types. IMTS may beused only for intra prediction blocks, or may be used for both intraprediction blocks and inter prediction block.

The three processes of MTS, SBT, and IMTS have been described above asselection processes for selectively switching transform types for use inorthogonal transform. However, all of the three selection processes maybe made effective, or only part of the selection processes may beselectively made effective. Whether each of the selection processes ismade effective can be identified based on flag information or the likein a header such as an SPS. For example, when all of the three selectionprocesses are effective, one of the three selection processes isselected for each CU and orthogonal transform of the CU is performed. Itis to be noted that the selection processes for selectively switchingthe transform types may be selection processes different from the abovethree selection processes, or each of the three selection processes maybe replaced by another process as long as at least one of the followingfour functions [1] to [4] can be achieved. Function [1] is a functionfor performing orthogonal transform of the entire CU and encodinginformation indicating the transform type used in the transform.Function [2] is a function for performing orthogonal transform of theentire CU and determining the transform type based on a predeterminedrule without encoding information indicating the transform type.Function [3] is a function for performing orthogonal transform of apartial region of a CU and encoding information indicating the transformtype used in the transform. Function [4] is a function for performingorthogonal transform of a partial region of a CU and determining thetransform type based on a predetermined rule without encodinginformation indicating the transform type used in the transform.

It is to be noted that whether each of MTS, IMTS, and SBT is applied maybe determined for each processing unit. For example, whether each ofMTS, IMTS, and SBT is applied may be determined for each sequence,picture, brick, slice, CTU, or CU.

It is to be noted that a tool for selectively switching transform typesin the present disclosure may be rephrased by a method for selectivelyselecting a basis for use in a transform process, a selection process,or a process for selecting a basis. In addition, the tool forselectively switching transform types may be rephrased by a mode foradaptively selecting a transform type.

FIG. 16 is a flow chart illustrating one example of a process performedby transformer 106.

For example, transformer 106 determines whether to perform orthogonaltransform (Step St_1). Here, when determining to perform orthogonaltransform (Yes in Step St_1), transformer 106 selects a transform typefor use in orthogonal transform from a plurality of transform types(Step St_2). Next, transformer 106 performs orthogonal transform byapplying the selected transform type to the prediction residual of acurrent block (Step St_3). Transformer 106 then outputs informationindicating the selected transform type to entropy encoder 110, so as toallow entropy encoder 110 to encode the information (Step St_4). On theother hand, when determining not to perform orthogonal transform (No inStep St_1), transformer 106 outputs information indicating that noorthogonal transform is performed, so as to allow entropy encoder 110 toencode the information (Step St_5). It is to be noted that whether toperform orthogonal transform in Step St_1 may be determined based on,for example, the size of a transform block, a prediction mode applied tothe CU, etc. Alternatively, orthogonal transform may be performed usinga predefined transform type without encoding information indicating thetransform type for use in orthogonal transform.

FIG. 17 is a flow chart illustrating another example of a processperformed by transformer 106. It is to be noted that the exampleillustrated in FIG. 17 is an example of orthogonal transform in the casewhere transform types for use in orthogonal transform are selectivelyswitched as in the case of the example illustrated in FIG. 16 .

As one example, a first transform type group may include DCT2, DST7, andDCT8. As another example, a second transform type group may includeDCT2. The transform types included in the first transform type group andthe transform types included in the second transform type group maypartly overlap with each other, or may be totally different from eachother.

More specifically, transformer 106 determines whether a transform sizeis smaller than or equal to a predetermined value (Step Su_1). Here,when determining that the transform size is smaller than or equal to thepredetermined value (Yes in Step Su_1), transformer 106 performsorthogonal transform of the prediction residual of the current blockusing the transform type included in the first transform type group(Step Su_2). Next, transformer 106 outputs information indicating thetransform type to be used among at least one transform type included inthe first transform type group to entropy encoder 110, so as to allowentropy encoder 110 to encode the information (Step Su_3). On the otherhand, when determining that the transform size is not smaller than orequal to the predetermined value (No in Step Su_1), transformer 106performs orthogonal transform of the prediction residual of the currentblock using the second transform type group (Step Su_4).

In Step Su_3, the information indicating the transform type for use inorthogonal transform may be information indicating a combination of thetransform type to be applied vertically in the current block and thetransform type to be applied horizontally in the current block. Thefirst type group may include only one transform type, and theinformation indicating the transform type for use in orthogonaltransform may not be encoded. The second transform type group mayinclude a plurality of transform types, and information indicating thetransform type for use in orthogonal transform among the one or moretransform types included in the second transform type group may beencoded.

Alternatively, a transform type may be determined based only on atransform size. It is to be noted that such determinations are notlimited to the determination as to whether the transform size is smallerthan or equal to the predetermined value, and other processes are alsopossible as long as the processes are for determining a transform typefor use in orthogonal transform based on the transform size.

[Quantizer]

Quantizer 108 quantizes the transform coefficients output fromtransformer 106. More specifically, quantizer 108 scans, in a determinedscanning order, the transform coefficients of the current block, andquantizes the scanned transform coefficients based on quantizationparameters (QP) corresponding to the transform coefficients. Quantizer108 then outputs the quantized transform coefficients (hereinafter alsoreferred to as quantized coefficients) of the current block to entropyencoder 110 and inverse quantizer 112.

A determined scanning order is an order for quantizing/inversequantizing transform coefficients. For example, a determined scanningorder is defined as ascending order of frequency (from low to highfrequency) or descending order of frequency (from high to lowfrequency).

A quantization parameter (QP) is a parameter defining a quantizationstep (quantization width). For example, when the value of thequantization parameter increases, the quantization step also increases.In other words, when the value of the quantization parameter increases,an error in quantized coefficients (quantization error) increases.

In addition, a quantization matrix may be used for quantization. Forexample, several kinds of quantization matrices may be usedcorrespondingly to frequency transform sizes such as 4×4 and 8×8,prediction modes such as intra prediction and inter prediction, andpixel components such as luma and chroma pixel components. It is to benoted that quantization means digitalizing values sampled atpredetermined intervals correspondingly to predetermined levels. In thistechnical field, quantization may be represented as other expressionssuch as rounding and scaling.

Methods using quantization matrices include a method using aquantization matrix which has been set directly at the encoder 100 sideand a method using a quantization matrix which has been set as a default(default matrix). At the encoder 100 side, a quantization matrixsuitable for features of an image can be set by directly setting aquantization matrix. This case, however, has a disadvantage ofincreasing a coding amount for encoding the quantization matrix. It isto be noted that a quantization matrix to be used to quantize thecurrent block may be generated based on a default quantization matrix oran encoded quantization matrix, instead of directly using the defaultquantization matrix or the encoded quantization matrix.

There is a method for quantizing a high-frequency coefficient and alow-frequency coefficient in the same manner without using aquantization matrix. It is to be noted that this method is equivalent toa method using a quantization matrix (flat matrix) whose allcoefficients have the same value.

The quantization matrix may be encoded, for example, at the sequencelevel, picture level, slice level, brick level, or CTU level.

When using a quantization matrix, quantizer 108 scales, for eachtransform coefficient, for example a quantization width which can becalculated based on a quantization parameter, etc., using the value ofthe quantization matrix. The quantization process performed withoutusing any quantization matrix may be a process of quantizing transformcoefficients based on a quantization width calculated based on aquantization parameter, etc. It is to be noted that, in the quantizationprocess performed without using any quantization matrix, thequantization width may be multiplied by a predetermined value which iscommon for all the transform coefficients in a block.

FIG. 18 is a block diagram illustrating one example of a configurationof quantizer 108.

For example, quantizer 108 includes difference quantization parametergenerator 108 a, predicted quantization parameter generator 108 b,quantization parameter generator 108 c, quantization parameter storage108 d, and quantization executor 108 e.

FIG. 19 is a flow chart illustrating one example of quantizationperformed by quantizer 108.

As one example, quantizer 108 may perform quantization for each CU basedon the flow chart illustrated in FIG. 19 . More specifically,quantization parameter generator 108 c determines whether to performquantization (Step Sv_1). Here, when determining to perform quantization(Yes in Step Sv_1), quantization parameter generator 108 c generates aquantization parameter for a current block (Step Sv_2), and stores thequantization parameter into quantization parameter storage 108 d (StepSv_3).

Next, quantization executor 108 e quantizes transform coefficients ofthe current block using the quantization parameter generated in StepSv_2 (Step Sv_4). Predicted quantization parameter generator 108 b thenobtains a quantization parameter for a processing unit different fromthe current block from quantization parameter storage 108 d (Step Sv_5).Predicted quantization parameter generator 108 b generates a predictedquantization parameter of the current block based on the obtainedquantization parameter (Step Sv_6). Difference quantization parametergenerator 108 a calculates the difference between the quantizationparameter of the current block generated by quantization parametergenerator 108 c and the predicted quantization parameter of the currentblock generated by predicted quantization parameter generator 108 b(Step Sv_7). The difference quantization parameter is generated bycalculating the difference. Difference quantization parameter generator108 a outputs the difference quantization parameter to entropy encoder110, so as to allow entropy encoder 110 to encode the differencequantization parameter (Step Sv_8).

It is to be noted that the difference quantization parameter may beencoded, for example, at the sequence level, picture level, slice level,brick level, or CTU level. In addition, the initial value of thequantization parameter may be encoded at the sequence level, picturelevel, slice level, brick level, or CTU level. At this time, thequantization parameter may be generated using the initial value of thequantization parameter and the difference quantization parameter.

It is to be noted that quantizer 108 may include a plurality ofquantizers, and may apply dependent quantization in which transformcoefficients are quantized using a quantization method selected from aplurality of quantization methods.

[Entropy Encoder]

FIG. 20 is a block diagram illustrating one example of a configurationof entropy encoder 110.

Entropy encoder 110 generates a stream by entropy encoding the quantizedcoefficients input from quantizer 108 and a prediction parameter inputfrom prediction parameter generator 130. For example, context-basedadaptive binary arithmetic coding (CABAC) is used as the entropyencoding. More specifically, entropy encoder 110 includes binarizer 110a, context controller 110 b, and binary arithmetic encoder 110 c.Binarizer 110 a performs binarization in which multi-level signals suchas quantized coefficients and a prediction parameter are transformedinto binary signals. Examples of binarization methods include truncatedRice binarization, exponential Golomb codes, and fixed lengthbinarization. Context controller 110 b derives a context value accordingto a feature or a surrounding state of a syntax element, that is, anoccurrence probability of a binary signal. Examples of methods forderiving a context value include bypass, referring to a syntax element,referring to an upper and left adjacent blocks, referring tohierarchical information, and others. Binary arithmetic encoder 110 carithmetically encodes the binary signal using the derived contextvalue.

FIG. 21 is a diagram illustrating a flow of CABAC in entropy encoder110.

First, initialization is performed in CABAC in entropy encoder 110. Inthe initialization, initialization in binary arithmetic encoder 110 cand setting of an initial context value are performed. For example,binarizer 110 a and binary arithmetic encoder 110 c execute binarizationand arithmetic encoding of a plurality of quantization coefficients in aCTU sequentially. At this time, context controller 110 b updates thecontext value each time arithmetic encoding is performed. Contextcontroller 110 b then saves the context value as a post process. Thesaved context value is used, for example, to initialize the contextvalue for the next CTU.

[Inverse Quantizer]

Inverse quantizer 112 inverse quantizes quantized coefficients whichhave been input from quantizer 108. More specifically, inverse quantizer112 inverse quantizes, in a determined scanning order, quantizedcoefficients of the current block. Inverse quantizer 112 then outputsthe inverse quantized transform coefficients of the current block toinverse transformer 114.

[Inverse Transformer]

Inverse transformer 114 restores prediction errors by inversetransforming the transform coefficients which have been input frominverse quantizer 112. More specifically, inverse transformer 114restores the prediction residuals of the current block by performing aninverse transform corresponding to the transform applied to thetransform coefficients by transformer 106. Inverse transformer 114 thenoutputs the restored prediction residuals to adder 116.

It is to be noted that since information is normally lost inquantization, the restored prediction residuals do not match theprediction errors calculated by subtractor 104. In other words, therestored prediction residuals normally include quantization errors.

[Adder]

Adder 116 reconstructs the current block by adding the predictionresiduals which have been input from inverse transformer 114 andprediction images which have been input from prediction controller 128.Consequently, a reconstructed image is generated. Adder 116 then outputsthe reconstructed image to block memory 118 and loop filter 120.

[Block Memory]

Block memory 118 is storage for storing a block which is included in acurrent picture and is referred to in intra prediction. Morespecifically, block memory 118 stores a reconstructed image output fromadder 116.

[Frame Memory]

Frame memory 122 is, for example, storage for storing reference picturesfor use in inter prediction, and is also referred to as a frame buffer.More specifically, frame memory 122 stores a reconstructed imagefiltered by loop filter 120.

[Loop Filter]

Loop filter 120 applies a loop filter to a reconstructed image output byadder 116, and outputs the filtered reconstructed image to frame memory122. A loop filter is a filter used in an encoding loop (in-loopfilter). Examples of loop filters include, for example, an adaptive loopfilter (ALF), a deblocking filter (DF or DBF), a sample adaptive offset(SAO), etc.

FIG. 22 is a block diagram illustrating one example of a configurationof loop filter 120.

For example, as illustrated in FIG. 22 , loop filter 120 includesdeblocking filter executor 120 a, SAO executor 120 b, and ALF executor120 c. Deblocking filter executor 120 a performs a deblocking filterprocess of the reconstructed image. SAO executor 120 b performs a SAOprocess of the reconstructed image after being subjected to thedeblocking filter process. ALF executor 120 c performs an ALF process ofthe reconstructed image after being subjected to the SAO process. TheALF and deblocking filter processes are described later in detail. TheSAO process is a process for enhancing image quality by reducing ringing(a phenomenon in which pixel values are distorted like waves around anedge) and correcting deviation in pixel value. Examples of SAO processesinclude an edge offset process and a band offset process. It is to benoted that loop filter 120 does not always need to include all theconstituent elements disclosed in FIG. 22 , and may include only part ofthe constituent elements. In addition, loop filter 120 may be configuredto perform the above processes in a processing order different from theone disclosed in FIG. 22 .

[Loop Filter>Adaptive Loop Filter]

In an ALF, a least square error filter for removing compressionartifacts is applied. For example, one filter selected from among aplurality of filters based on the direction and activity of localgradients is applied for each of 2×2 pixel sub-blocks in the currentblock.

More specifically, first, each sub-block (for example, each 2×2 pixelsub-block) is categorized into one out of a plurality of classes (forexample, fifteen or twenty-five classes). The categorization of thesub-block is based on, for example, gradient directionality andactivity. In a specific example, category index C (for example, C=5D+A)is calculated based on gradient directionality D (for example, 0 to 2 or0 to 4) and gradient activity A (for example, 0 to 4). Then, based oncategory index C, each sub-block is categorized into one out of aplurality of classes.

For example, gradient directionality D is calculated by comparinggradients of a plurality of directions (for example, the horizontal,vertical, and two diagonal directions). Moreover, for example, gradientactivity A is calculated by adding gradients of a plurality ofdirections and quantizing the result of the addition.

The filter to be used for each sub-block is determined from among theplurality of filters based on the result of such categorization.

The filter shape to be used in an ALF is, for example, a circularsymmetric filter shape. FIG. 23A through FIG. 23C illustrate examples offilter shapes used in ALFs. FIG. 23A illustrates a 5×5 diamond shapefilter, FIG. 23B illustrates a 7×7 diamond shape filter, and FIG. 23Cillustrates a 9×9 diamond shape filter. Information indicating thefilter shape is normally signaled at the picture level. It is to benoted that the signaling of such information indicating the filter shapedoes not necessarily need to be performed at the picture level, and maybe performed at another level (for example, at the sequence level, slicelevel, brick level, CTU level, or CU level).

The ON or OFF of the ALF is determined, for example, at the picturelevel or CU level. For example, the decision of whether to apply the ALFto luma may be made at the CU level, and the decision of whether toapply ALF to chroma may be made at the picture level. Informationindicating ON or OFF of the ALF is normally signaled at the picturelevel or CU level. It is to be noted that the signaling of informationindicating ON or OFF of the ALF does not necessarily need to beperformed at the picture level or CU level, and may be performed atanother level (for example, at the sequence level, slice level, bricklevel, or CTU level).

In addition, as described above, one filter is selected from theplurality of filters, and an ALF process of a sub-block is performed. Acoefficient set of coefficients to be used for each of the plurality offilters (for example, up to the fifteenth or twenty-fifth filter) isnormally signaled at the picture level. It is to be noted that thecoefficient set does not always need to be signaled at the picturelevel, and may be signaled at another level (for example, the sequencelevel, slice level, brick level, CTU level, CU level, or sub-blocklevel).

[Loop Filter>Cross Component Adaptive Loop Filter]

FIG. 23D is a diagram illustrating an example where Y samples (firstcomponent) are used for a cross component ALF (CCALF) for Cb and a CCALFfor Cr (components different from the first component). FIG. 23E is adiagram illustrating a diamond shaped filter.

One example of CC-ALF operates by applying a linear, diamond shapedfilter (FIGS. 23D, 23E) to a luma channel for each chroma component. Thefilter coefficients, for example, may be transmitted in the APS, scaledby a factor of 2{circumflex over ( )}10, and rounded for fixed pointrepresentation. The application of the filters is controlled on avariable block size and signaled by a context-coded flag received foreach block of samples. The block size along with a CC-ALF enabling flagis received at the slice-level for each chroma component. Syntax andsemantics for CC-ALF are provided in the Appendix. In the contribution,the following block sizes (in chroma samples) were supported: 16×16,32×32, 64×64, and 128×128.

[Loop Filter>Joint Chroma Cross Component Adaptive Loop Filter]

FIG. 23F is a diagram illustrating an example for a joint chroma CCALF(JC-CCALF).

One example of JC-CCALF, where only one CCALF filter will be used togenerate one CCALF filtered output as a chroma refinement signal for onecolor component only, while a properly weighted version of the samechroma refinement signal will be applied to the other color component.In this way, the complexity of existing CCALF is reduced roughly byhalf.

The weight value is coded into a sign flag and a weight index. Theweight index (denoted as weight_index) is coded into 3 bits, andspecifies the magnitude of the JC-CCALF weight JcCcWeight. It cannot beequal to 0. The magnitude of JcCcWeight is determined as follows.

-   -   If weight_index is less than or equal to 4, JcCcWeight is equal        to weight_index>>2.    -   Otherwise, JcCcWeight is equal to 4/(weight_index−4).

The block-level on/off control of ALF filtering for Cb and Cr areseparate. This is the same as in CCALF, and two separate sets ofblock-level on/off control flags will be coded. Different from CCALF,herein, the Cb, Cr on/off control block sizes are the same, and thus,only one block size variable is coded.

[Loop Filter>Deblocking Filter]

In a deblocking filter process, loop filter 120 performs a filterprocess on a block boundary in a reconstructed image so as to reducedistortion which occurs at the block boundary.

FIG. 24 is a block diagram illustrating one example of a specificconfiguration of deblocking filter executor 120 a.

For example, deblocking filter executor 120 a includes: boundarydeterminer 1201; filter determiner 1203; filter executor 1205; processdeterminer 1208; filter characteristic determiner 1207; and switches1202, 1204, and 1206.

Boundary determiner 1201 determines whether a pixel to be deblockfiltered (that is, a current pixel) is present around a block boundary.Boundary determiner 1201 then outputs the determination result to switch1202 and process determiner 1208.

In the case where boundary determiner 1201 has determined that a currentpixel is present around a block boundary, switch 1202 outputs anunfiltered image to switch 1204. In the opposite case where boundarydeterminer 1201 has determined that no current pixel is present around ablock boundary, switch 1202 outputs an unfiltered image to switch 1206.It is to be noted that the unfiltered image is an image configured witha current pixel and at least one surrounding pixel located around thecurrent pixel.

Filter determiner 1203 determines whether to perform deblockingfiltering of the current pixel, based on the pixel value of at least onesurrounding pixel located around the current pixel. Filter determiner1203 then outputs the determination result to switch 1204 and processdeterminer 1208.

In the case where filter determiner 1203 has determined to performdeblocking filtering of the current pixel, switch 1204 outputs theunfiltered image obtained through switch 1202 to filter executor 1205.In the opposite case where filter determiner 1203 has determined not toperform deblocking filtering of the current pixel, switch 1204 outputsthe unfiltered image obtained through switch 1202 to switch 1206.

When obtaining the unfiltered image through switches 1202 and 1204,filter executor 1205 executes, for the current pixel, deblockingfiltering having the filter characteristic determined by filtercharacteristic determiner 1207. Filter executor 1205 then outputs thefiltered pixel to switch 1206.

Under control by process determiner 1208, switch 1206 selectivelyoutputs a pixel which has not been deblock filtered and a pixel whichhas been deblock filtered by filter executor 1205.

Process determiner 1208 controls switch 1206 based on the results ofdeterminations made by boundary determiner 1201 and filter determiner1203. In other words, process determiner 1208 causes switch 1206 tooutput the pixel which has been deblock filtered when boundarydeterminer 1201 has determined that the current pixel is present aroundthe block boundary and filter determiner 1203 has determined to performdeblocking filtering of the current pixel. In addition, in a case otherthan the above case, process determiner 1208 causes switch 1206 tooutput the pixel which has not been deblock filtered. A filtered imageis output from switch 1206 by repeating output of a pixel in this way.It is to be noted that the configuration illustrated in FIG. 24 is oneexample of a configuration in deblocking filter executor 120 a.Deblocking filter executor 120 a may have another configuration.

FIG. 25 is a diagram illustrating an example of a deblocking filterhaving a symmetrical filtering characteristic with respect to a blockboundary.

In a deblocking filter process, one of two deblocking filters havingdifferent characteristics, that is, a strong filter and a weak filter isselected using pixel values and quantization parameters, for example. Inthe case of the strong filter, pixels p0 to p2 and pixels q0 to q2 arepresent across a block boundary as illustrated in FIG. 25 , the pixelvalues of the respective pixels q0 to q2 are changed to pixel values q′0to q′2 by performing computations according to the expressions below.q′0=(p1+2×p0+2×q0+2×q1+q2+4)/8q′1=(p0+q0+q1+q2+2)/4q′2=(p0+q0+q1+3×q2+2×q3+4)/8

It is to be noted that, in the above expressions, p0 to p2 and q0 to q2are the pixel values of respective pixels p0 to p2 and pixels q0 to q2.In addition, q3 is the pixel value of neighboring pixel q3 located atthe opposite side of pixel q2 with respect to the block boundary. Inaddition, in the right side of each of the expressions, coefficientswhich are multiplied with the respective pixel values of the pixels tobe used for deblocking filtering are filter coefficients.

Furthermore, in the deblocking filtering, clipping may be performed sothat the calculated pixel values do not change over a threshold value.In the clipping process, the pixel values calculated according to theabove expressions are clipped to a value obtained according to “apre-computation pixel value ±2×a threshold value” using the thresholdvalue determined based on a quantization parameter. In this way, it ispossible to prevent excessive smoothing.

FIG. 26 is a diagram for illustrating one example of a block boundary onwhich a deblocking filter process is performed. FIG. 27 is a diagramillustrating examples of Bs values.

The block boundary on which the deblocking filter process is performedis, for example, a boundary between CUs, PUs, or TUs having 8×8 pixelblocks as illustrated in FIG. 26 . The deblocking filter process isperformed, for example, in units of four rows or four columns. First,boundary strength (Bs) values are determined as indicated in FIG. 27 forblock P and block Q illustrated in FIG. 26 .

According to the Bs values in FIG. 27 , whether to perform deblockingfilter processes of block boundaries belonging to the same image usingdifferent strengths may be determined. The deblocking filter process fora chroma signal is performed when a Bs value is 2. The deblocking filterprocess for a luma signal is performed when a Bs value is 1 or more anda determined condition is satisfied. It is to be noted that conditionsfor determining Bs values are not limited to those indicated in FIG. 27, and a Bs value may be determined based on another parameter.

[Predictor (Intra Predictor, Inter Predictor, Prediction Controller)]

FIG. 28 is a flow chart illustrating one example of a process performedby a predictor of encoder 100. It is to be noted that the predictor, asone example, includes all or part of the following constituent elements:intra predictor 124; inter predictor 126; and prediction controller 128.The prediction executor includes, for example, intra predictor 124 andinter predictor 126.

The predictor generates a prediction image of a current block (StepSb_1). It is to be noted that the prediction image is, for example, anintra prediction image (intra prediction signal) or an inter predictionimage (inter prediction signal). More specifically, the predictorgenerates the prediction image of the current block using areconstructed image which has been already obtained for another blockthrough generation of a prediction image, generation of a predictionresidual, generation of quantized coefficients, restoring of aprediction residual, and addition of a prediction image.

The reconstructed image may be, for example, an image in a referencepicture or an image of an encoded block (that is, the other blockdescribed above) in a current picture which is the picture including thecurrent block. The encoded block in the current picture is, for example,a neighboring block of the current block.

FIG. 29 is a flow chart illustrating another example of a processperformed by the predictor of encoder 100.

The predictor generates a prediction image using a first method (StepSc_1 a), generates a prediction image using a second method (Step Sc_1b), and generates a prediction image using a third method (Step Sc_1 c).The first method, the second method, and the third method may bemutually different methods for generating a prediction image. Each ofthe first to third methods may be an inter prediction method, an intraprediction method, or another prediction method. The above-describedreconstructed image may be used in these prediction methods.

Next, the predictor evaluates the prediction images generated in StepsSc_1 a, Sc_1 b, and Sc_1 c (Step Sc_2). For example, the predictorcalculates costs C for the prediction images generated in Step Sc_1 a,Sc_1 b, and Sc_1 c, and evaluates the prediction images by comparing thecosts C of the prediction images. It is to be noted that cost C iscalculated according to an expression of an R-D optimization model, forexample, C=D+λ×R. In this expression, D indicates compression artifactsof a prediction image, and is represented as, for example, a sum ofabsolute differences between the pixel value of a current block and thepixel value of a prediction image. In addition, R indicates a bit rateof a stream. In addition, a indicates, for example, a multiplieraccording to the method of Lagrange multiplier.

The predictor then selects one of the prediction images generated inSteps Sc_1 a, Sc_1 b, and Sc_1 c (Step Sc_3). In other words, thepredictor selects a method or a mode for obtaining a final predictionimage. For example, the predictor selects the prediction image havingthe smallest cost C, based on costs C calculated for the predictionimages. Alternatively, the evaluation in Step Sc_2 and the selection ofthe prediction image in Step Sc_3 may be made based on a parameter whichis used in an encoding process. Encoder 100 may transform informationfor identifying the selected prediction image, the method, or the modeinto a stream. The information may be, for example, a flag or the like.In this way, decoder 200 is capable of generating a prediction imageaccording to the method or the mode selected by encoder 100, based onthe information. It is to be noted that, in the example illustrated inFIG. 29 , the predictor selects any of the prediction images after theprediction images are generated using the respective methods. However,the predictor may select a method or a mode based on a parameter for usein the above-described encoding process before generating predictionimages, and may generate a prediction image according to the method ormode selected.

For example, the first method and the second method may be intraprediction and inter prediction, respectively, and the predictor mayselect a final prediction image for a current block from predictionimages generated according to the prediction methods.

FIG. 30 is a flow chart illustrating another example of a processperformed by the predictor of encoder 100.

First, the predictor generates a prediction image using intra prediction(Step Sd_1 a), and generates a prediction image using inter prediction(Step Sd_1 b). It is to be noted that the prediction image generated byintra prediction is also referred to as an intra prediction image, andthe prediction image generated by inter prediction is also referred toas an inter prediction image.

Next, the predictor evaluates each of the intra prediction image and theinter prediction image (Step Sd_2). Cost C described above may be usedin the evaluation. The predictor may then select the prediction imagefor which the smallest cost C has been calculated among the intraprediction image and the inter prediction image, as the final predictionimage for the current block (Step Sd_3). In other words, the predictionmethod or the mode for generating the prediction image for the currentblock is selected.

[Intra Predictor]

Intra predictor 124 generates a prediction image (that is, intraprediction image) of a current block by performing intra prediction(also referred to as intra frame prediction) of the current block byreferring to a block or blocks in the current picture which is or arestored in block memory 118. More specifically, intra predictor 124generates an intra prediction image by performing intra prediction byreferring to pixel values (for example, luma and/or chroma values) of ablock or blocks neighboring the current block, and then outputs theintra prediction image to prediction controller 128.

For example, intra predictor 124 performs intra prediction by using onemode from among a plurality of intra prediction modes which have beenpredefined. The intra prediction modes normally include one or morenon-directional prediction modes and a plurality of directionalprediction modes.

The one or more non-directional prediction modes include, for example,planar prediction mode and DC prediction mode defined in the H.265/HEVCstandard.

The plurality of directional prediction modes include, for example, thethirty-three directional prediction modes defined in the H.265/HEVCstandard. It is to be noted that the plurality of directional predictionmodes may further include thirty-two directional prediction modes inaddition to the thirty-three directional prediction modes (for a totalof sixty-five directional prediction modes). FIG. 31 is a diagramillustrating sixty-seven intra prediction modes in total used in intraprediction (two non-directional prediction modes and sixty-fivedirectional prediction modes). The solid arrows represent thethirty-three directions defined in the H.265/HEVC standard, and thedashed arrows represent the additional thirty-two directions (the twonon-directional prediction modes are not illustrated in FIG. 31 ).

In various kinds of mounting examples, a luma block may be referred toin intra prediction of a chroma block. In other words, a chromacomponent of the current block may be predicted based on a lumacomponent of the current block. Such intra prediction is also referredto as cross-component linear model (CCLM). The intra prediction mode fora chroma block in which such a luma block is referred to (also referredto as, for example, a CCLM mode) may be added as one of the intraprediction modes for chroma blocks.

Intra predictor 124 may correct intra-predicted pixel values based onhorizontal/vertical reference pixel gradients. The intra predictionwhich accompanies this sort of correcting is also referred to asposition dependent intra prediction combination (PDPC). Informationindicating whether to apply PDPC (referred to as, for example, a PDPCflag) is normally signaled at the CU level. It is to be noted that thesignaling of such information does not necessarily need to be performedat the CU level, and may be performed at another level (for example, atthe sequence level, picture level, slice level, brick level, or CTUlevel).

FIG. 32 is a flow chart illustrating one example of a process performedby intra predictor 124.

Intra predictor 124 selects one intra prediction mode from a pluralityof intra prediction modes (Step Sw_1). Intra predictor 124 thengenerates a prediction image according to the selected intra predictionmode (Step Sw_2). Next, intra predictor 124 determines most probablemodes (MPMs) (Step Sw_3). MPMs include, for example, six intraprediction modes. Two modes among the six intra prediction modes may beplanar mode and DC prediction mode, and the other four modes may bedirectional prediction modes. Intra predictor 124 determines whether theintra prediction mode selected in Step Sw_1 is included in the MPMs(Step Sw_4).

Here, when determining that the intra prediction mode selected in StepSw_1 is included in the MPMs (Yes in Step Sw_4), intra predictor 124sets an MPM flag to 1 (Step Sw_5), and generates information indicatingthe selected intra prediction mode among the MPMs (Step Sw_6). It is tobe noted that the MPM flag set to 1 and the information indicating theintra prediction mode are encoded as prediction parameters by entropyencoder 110.

When determining that the selected intra prediction mode is not includedin the MPMs (No in Step Sw_4), intra predictor 124 sets the MPM flag to0 (Step Sw_7). Alternatively, intra predictor 124 does not set any MPMflag. Intra predictor 124 then generates information indicating theselected intra prediction mode among at least one intra prediction modewhich is not included in the MPMs (Step Sw_8). It is to be noted thatthe MPM flag set to 0 and the information indicating the intraprediction mode are encoded as prediction parameters by entropy encoder110. The information indicating the intra prediction mode indicates, forexample, any one of 0 to 60.

[Inter Predictor]

Inter predictor 126 generates a prediction image (inter predictionimage) by performing inter prediction (also referred to as inter frameprediction) of the current block by referring to a block or blocks in areference picture which is different from the current picture and isstored in frame memory 122. Inter prediction is performed in units of acurrent block or a current sub-block in the current block. The sub-blockis included in the block and is a unit smaller than the block. The sizeof the sub-block may be 4×4 pixels, 8×8 pixels, or another size. Thesize of the sub-block may be switched for a unit such as slice, brick,picture, etc.

For example, inter predictor 126 performs motion estimation in areference picture for a current block or a current sub-block, and findsout a reference block or a reference sub-block which best matches thecurrent block or current sub-block. Inter predictor 126 then obtainsmotion information (for example, a motion vector) which compensates amotion or a change from the reference block or the reference sub-blockto the current block or the current sub-block. Inter predictor 126generates an inter prediction image of the current block or the currentsub-block by performing motion compensation (or motion prediction) basedon the motion information. Inter predictor 126 outputs the generatedinter prediction image to prediction controller 128.

The motion information used in motion compensation may be signaled asinter prediction images in various forms. For example, a motion vectormay be signaled. As another example, the difference between a motionvector and a motion vector predictor may be signaled.

[Reference Picture List]

FIG. 33 is a diagram illustrating examples of reference pictures. FIG.34 is a conceptual diagram illustrating examples of reference picturelists. Each reference picture list is a list indicating at least onereference picture stored in frame memory 122. It is to be noted that, inFIG. 33 , each of rectangles indicates a picture, each of arrowsindicates a picture reference relationship, the horizontal axisindicates time, I, P, and B in the rectangles indicate an intraprediction picture, a uni-prediction picture, and a bi-predictionpicture, respectively, and numerals in the rectangles indicate adecoding order. As illustrated in FIG. 33 , the decoding order of thepictures is an order of I0, P1, B2, B3, and B4, and the display order ofthe pictures is an order of I0, B3, B2, B4, and P1. As illustrated inFIG. 34 , the reference picture list is a list representing referencepicture candidates. For example, one picture (or a slice) may include atleast one reference picture list. For example, one reference picturelist is used when a current picture is a uni-prediction picture, and tworeference picture lists are used when a current picture is abi-prediction picture. In the examples of FIGS. 33 and 34 , picture B3which is current picture currPic has two reference picture lists whichare the L0 list and the L1 list. When current picture currPic is pictureB3, reference picture candidates for current picture currPic are I0, P1,and B2, and the reference picture lists (which are the L0 list and theL1 list) indicate these pictures. Inter predictor 126 or predictioncontroller 128 specifies which picture in each reference picture list isto be actually referred to in form of a reference picture indexrefIdxLx. In FIG. 34 , reference pictures P1 and B2 are specified byreference picture indices refIdxL0 and refIdxL1.

Such a reference picture list may be generated for each unit such as asequence, picture, slice, brick, CTU, or CU. In addition, amongreference pictures indicated in reference picture lists, a referencepicture index indicating a reference picture to be referred to in interprediction may be signaled at the sequence level, picture level, slicelevel, brick level, CTU level, or CU level. In addition, a commonreference picture list may be used in a plurality of inter predictionmodes.

[Basic Flow of Inter Prediction]

FIG. 35 is a flow chart illustrating a basic processing flow of interprediction.

First, inter predictor 126 generates a prediction signal (Steps Se_1 toSe_3). Next, subtractor 104 generates the difference between a currentblock and a prediction image as a prediction residual (Step Se_4).

Here, in the generation of the prediction image, inter predictor 126generates the prediction image through, for example, determination of amotion vector (MV) of the current block (Steps Se_1 and Se_2) and motioncompensation (Step Se_3). Furthermore, in determination of an MV, interpredictor 126 determines the MV through, for example, selection of amotion vector candidate (MV candidate) (Step Se_1) and derivation of anMV (Step Se_2). The selection of the MV candidate is made by means of,for example, inter predictor 126 generating an MV candidate list andselecting at least one MV candidate from the MV candidate list. It is tobe noted that MVs derived in the past may be added to the MV candidatelist. Alternatively, in derivation of an MV, inter predictor 126 mayfurther select at least one MV candidate from the at least one MVcandidate, and determine the selected at least one MV candidate as theMV for the current block. Alternatively, inter predictor 126 maydetermine the MV for the current block by performing estimation in areference picture region specified by each of the selected at least oneMV candidate. It is to be noted that the estimation in the referencepicture region may be referred to as motion estimation.

In addition, although Steps Se_1 to Se_3 are performed by interpredictor 126 in the above-described example, a process that is, forexample, Step Se_1, Step Se_2, or the like may be performed by anotherconstituent element included in encoder 100.

It is to be noted that an MV candidate list may be generated for eachprocess in inter prediction mode, or a common MV candidate list may beused in a plurality of inter prediction modes. The processes in StepsSe_3 and Se_4 correspond to Steps Sa_3 and Sa_4 illustrated in FIG. 9 ,respectively. The process in Step Se_3 corresponds to the process inStep Sd_1 b in FIG. 30 .

[MV Derivation Flow]

FIG. 36 is a flow chart illustrating one example of MV derivation.

Inter predictor 126 may derive an MV for a current block in a mode forencoding motion information (for example, an MV). In this case, forexample, the motion information may be encoded as a predictionparameter, and may be signaled. In other words, the encoded motioninformation is included in a stream.

Alternatively, inter predictor 126 may derive an MV in a mode in whichmotion information is not encoded. In this case, no motion informationis included in the stream.

Here, MV derivation modes include a normal inter mode, a normal mergemode, a FRUC mode, an affine mode, etc. which are described later. Modesin which motion information is encoded among the modes include thenormal inter mode, the normal merge mode, the affine mode (specifically,an affine inter mode and an affine merge mode), etc. It is to be notedthat motion information may include not only an MV but also MV predictorselection information which is described later. Modes in which no motioninformation is encoded include the FRUC mode, etc. Inter predictor 126selects a mode for deriving an MV of the current block from theplurality of modes, and derives the MV of the current block using theselected mode.

FIG. 37 is a flow chart illustrating another example of MV derivation.

Inter predictor 126 may derive an MV for a current block in a mode inwhich an MV difference is encoded. In this case, for example, the MVdifference is encoded as a prediction parameter, and is signaled. Inother words, the encoded MV difference is included in a stream. The MVdifference is the difference between the MV of the current block and theMV predictor. It is to be noted that the MV predictor is a motion vectorpredictor.

Alternatively, inter predictor 126 may derive an MV in a mode in whichno MV difference is encoded. In this case, no encoded MV difference isincluded in the stream.

Here, as described above, the MV derivation modes include the normalinter mode, the normal merge mode, the FRUC mode, the affine mode, etc.which are described later. Modes in which an MV difference is encodedamong the modes include the normal inter mode, the affine mode(specifically, the affine inter mode), etc. Modes in which no MVdifference is encoded include the FRUC mode, the normal merge mode, theaffine mode (specifically, the affine merge mode), etc. Inter predictor126 selects a mode for deriving an MV of the current block from theplurality of modes, and derives the MV for the current block using theselected mode.

[MV Derivation Modes]

FIGS. 38A and 38B are each a diagram illustrating one example ofcategorization of modes for MV derivation. For example, as illustratedin FIG. 38A, MV derivation modes are roughly categorized into threemodes according to whether to encode motion information and whether toencode MV differences. The three modes are inter mode, merge mode, andframe rate up-conversion (FRUC) mode. The inter mode is a mode in whichmotion estimation is performed, and in which motion information and anMV difference are encoded. For example, as illustrated in FIG. 38B, theinter mode includes affine inter mode and normal inter mode. The mergemode is a mode in which no motion estimation is performed, and in whichan MV is selected from an encoded surrounding block and an MV for thecurrent block is derived using the MV. The merge mode is a mode inwhich, basically, motion information is encoded and no MV difference isencoded. For example, as illustrated in FIG. 38B, the merge modesinclude normal merge mode (also referred to as normal merge mode orregular merge mode), merge with motion vector difference (MMVD) mode,combined inter merge/intra prediction (CIIP) mode, triangle mode, ATMVPmode, and affine merge mode. Here, an MV difference is encodedexceptionally in the MMVD mode among the modes included in the mergemodes. It is to be noted that the affine merge mode and the affine intermode are modes included in the affine modes. The affine mode is a modefor deriving, as an MV of a current block, an MV of each of a pluralityof sub-blocks included in the current block, assuming affine transform.The FRUC mode is a mode which is for deriving an MV of the current blockby performing estimation between encoded regions, and in which neithermotion information nor any MV difference is encoded. It is to be notedthat the respective modes will be described later in detail.

It is to be noted that the categorization of the modes illustrated inFIGS. 38A and 38B are examples, and categorization is not limitedthereto. For example, when an MV difference is encoded in CIIP mode, theCIIP mode is categorized into inter modes.

[MV Derivation>Normal Inter Mode]

The normal inter mode is an inter prediction mode for deriving an MV ofa current block by finding out a block similar to the image of thecurrent block from a reference picture region specified by an MVcandidate. In this normal inter mode, an MV difference is encoded.

FIG. 39 is a flow chart illustrating an example of inter prediction bynormal inter mode.

First, inter predictor 126 obtains a plurality of MV candidates for acurrent block based on information such as MVs of a plurality of encodedblocks temporally or spatially surrounding the current block (StepSg_1). In other words, inter predictor 126 generates an MV candidatelist.

Next, inter predictor 126 extracts N (an integer of 2 or larger) MVcandidates from the plurality of MV candidates obtained in Step Sg_1, asmotion vector predictor candidates according to a predetermined priorityorder (Step Sg_2). It is to be noted that the priority order isdetermined in advance for each of the N MV candidates.

Next, inter predictor 126 selects one MV predictor candidate from the NMV predictor candidates as the MV predictor for the current block (StepSg_3). At this time, inter predictor 126 encodes, in a stream, MVpredictor selection information for identifying the selected MVpredictor. In other words, inter predictor 126 outputs the MV predictorselection information as a prediction parameter to entropy encoder 110through prediction parameter generator 130.

Next, inter predictor 126 derives an MV of a current block by referringto an encoded reference picture (Step Sg_4). At this time, interpredictor 126 further encodes, in the stream, the difference valuebetween the derived MV and the MV predictor as an MV difference. Inother words, inter predictor 126 outputs the MV difference as aprediction parameter to entropy encoder 110 through prediction parametergenerator 130. It is to be noted that the encoded reference picture is apicture including a plurality of blocks which have been reconstructedafter being encoded.

Lastly, inter predictor 126 generates a prediction image for the currentblock by performing motion compensation of the current block using thederived MV and the encoded reference picture (Step Sg_5). The processesin Steps Sg_1 to Sg_5 are executed on each block. For example, when theprocesses in Steps Sg_1 to Sg_5 are executed on each of all the blocksin the slice, inter prediction of the slice using the normal inter modefinishes. For example, when the processes in Steps Sg_1 to Sg_5 areexecuted on each of all the blocks in the picture, inter prediction ofthe picture using the normal inter mode finishes. It is to be noted thatnot all the blocks included in the slice may be subjected to theprocesses in Steps Sg_1 to Sg_5, and inter prediction of the slice usingthe normal inter mode may finish when part of the blocks are subjectedto the processes. Likewise, inter prediction of the picture using thenormal inter mode may finish when the processes in Steps Sg_1 to Sg_5are executed on part of the blocks in the picture.

It is to be noted that the prediction image is an inter predictionsignal as described above. In addition, information indicating the interprediction mode (normal inter mode in the above example) used togenerate the prediction image is, for example, encoded as a predictionparameter in an encoded signal.

It is to be noted that the MV candidate list may be also used as a listfor use in another mode. In addition, the processes related to the MVcandidate list may be applied to processes related to the list for usein another mode. The processes related to the MV candidate list include,for example, extraction or selection of an MV candidate from the MVcandidate list, reordering of MV candidates, or deletion of an MVcandidate.

[MV Derivation>Normal Merge Mode]

The normal merge mode is an inter prediction mode for selecting an MVcandidate from an MV candidate list as an MV for a current block,thereby deriving the MV. It is to be noted that the normal merge mode isa merge mode in a narrow meaning and is also simply referred to as amerge mode. In this embodiment, the normal merge mode and the merge modeare distinguished, and the merge mode is used in a broad meaning.

FIG. 40 is a flow chart illustrating an example of inter prediction bynormal merge mode.

First, inter predictor 126 obtains a plurality of MV candidates for acurrent block based on information such as MVs of a plurality of encodedblocks temporally or spatially surrounding the current block (StepSh_1). In other words, inter predictor 126 generates an MV candidatelist.

Next, inter predictor 126 selects one MV candidate from the plurality ofMV candidates obtained in Step Sh_1, thereby deriving an MV for thecurrent block (Step Sh_2). At this time, inter predictor 126 encodes, ina stream, MV selection information for identifying the selected MVcandidate. In other words, inter predictor 126 outputs the MV selectioninformation as a prediction parameter to entropy encoder 110 throughprediction parameter generator 130.

Lastly, inter predictor 126 generates a prediction image for the currentblock by performing motion compensation of the current block using thederived MV and the encoded reference picture (Step Sh_3). The processesin Steps Sh_1 to Sh_3 are executed, for example, on each block. Forexample, when the processes in Steps Sh_1 to Sh_3 are executed on eachof all the blocks in the slice, inter prediction of the slice using thenormal merge mode finishes. In addition, when the processes in StepsSh_1 to Sh_3 are executed on each of all the blocks in the picture,inter prediction of the picture using the normal merge mode finishes. Itis to be noted that not all the blocks included in the slice may besubjected to the processes in Steps Sh_1 to Sh_3, and inter predictionof the slice using the normal merge mode may finish when part of theblocks are subjected to the processes. Likewise, inter prediction of thepicture using the normal merge mode may finish when the processes inSteps Sh_1 to Sh_3 are executed on part of the blocks in the picture.

In addition, information indicating the inter prediction mode (normalmerge mode in the above example) used to generate the prediction imageis, for example, encoded as a prediction parameter in a stream.

FIG. 41 is a diagram for illustrating one example of an MV derivationprocess for a current picture by normal merge mode.

First, inter predictor 126 generates an MV candidate list in which MVcandidates are registered. Examples of MV candidates include: spatiallyneighboring MV candidates which are MVs of a plurality of encoded blockslocated spatially surrounding a current block; temporally neighboring MVcandidates which are MVs of surrounding blocks on which the position ofa current block in an encoded reference picture is projected; combinedMV candidates which are MVs generated by combining the MV value of aspatially neighboring MV predictor and the MV value of a temporallyneighboring MV predictor; and a zero MV candidate which is an MV havinga zero value.

Next, inter predictor 126 selects one MV candidate from a plurality ofMV candidates registered in an MV candidate list, and determines the MVcandidate as the MV of the current block.

Furthermore, entropy encoder 110 writes and encodes, in a stream,merge_idx which is a signal indicating which MV candidate has beenselected.

It is to be noted that the MV candidates registered in the MV candidatelist described in FIG. 41 are examples. The number of MV candidates maybe different from the number of MV candidates in the diagram, the MVcandidate list may be configured in such a manner that some of the kindsof the MV candidates in the diagram may not be included, or that one ormore MV candidates other than the kinds of MV candidates in the diagramare included.

A final MV may be determined by performing a dynamic motion vectorrefreshing (DMVR) to be described later using the MV of the currentblock derived by normal merge mode. It is to be noted that, in normalmerge mode, no MV difference is encoded, but an MV difference isencoded. In MMVD mode, one MV candidate is selected from an MV candidatelist as in the case of normal merge mode, an MV difference is encoded.As illustrated in FIG. 38B, MMVD may be categorized into merge modestogether with normal merge mode. It is to be noted that the MVdifference in MMVD mode does not always need to be the same as the MVdifference for use in inter mode. For example, MV difference derivationin MMVD mode may be a process that requires a smaller amount ofprocessing than the amount of processing required for MV differencederivation in inter mode.

In addition, a combined inter merge/intra prediction (CIIP) mode may beperformed. The mode is for overlapping a prediction image generated ininter prediction and a prediction image generated in intra prediction togenerate a prediction image for a current block.

It is to be noted that the MV candidate list may be referred to as acandidate list. In addition, merge_idx is MV selection information.

[MV Derivation>HMVP Mode]

FIG. 42 is a diagram for illustrating one example of an MV derivationprocess for a current picture by HMVP merge mode.

In normal merge mode, an MV for, for example, a CU which is a currentblock is determined by selecting one MV candidate from an MV candidatelist generated by referring to an encoded block (for example, a CU).Here, another MV candidate may be registered in the MV candidate list.The mode in which such another MV candidate is registered is referred toas HMVP mode.

In HMVP mode, MV candidates are managed using a first-in first-out(FIFO) buffer for HMVP, separately from the MV candidate list for normalmerge mode.

In FIFO buffer, motion information such as MVs of blocks processed inthe past are stored newest first. In the management of the FIFO buffer,each time when one block is processed, the MV for the newest block (thatis the CU processed immediately before) is stored in the FIFO buffer,and the MV of the oldest CU (that is, the CU processed earliest) isdeleted from the FIFO buffer. In the example illustrated in FIG. 42 ,HMVP1 is the MV for the newest block, and HMVP5 is the MV for the oldestMV.

Inter predictor 126 then, for example, checks whether each MV managed inthe FIFO buffer is an MV different from all the MV candidates which havebeen already registered in the MV candidate list for normal merge modestarting from HMVP1. When determining that the MV is different from allthe MV candidates, inter predictor 126 may add the MV managed in theFIFO buffer in the MV candidate list for normal merge mode as an MVcandidate. At this time, the MV candidate registered from the FIFObuffer may be one or more.

By using the HMVP mode in this way, it is possible to add not only theMV of a block which neighbors the current block spatially or temporallybut also an MV for a block processed in the past. As a result, thevariation of MV candidates for normal merge mode is expanded, whichincreases the probability that coding efficiency can be increased.

It is to be noted that the MV may be motion information. In other words,information stored in the MV candidate list and the FIFO buffer mayinclude not only MV values but also reference picture information,reference directions, the numbers of pictures, etc. In addition, theblock is, for example, a CU.

It is to be noted that the MV candidate list and the FIFO bufferillustrated in FIG. 42 are examples. The MV candidate list and FIFObuffer may be different in size from those in FIG. 42 , or may beconfigured to register MV candidates in an order different from the onein FIG. 42 . In addition, the process described here is common betweenencoder 100 and decoder 200.

It is to be noted that the HMVP mode can be applied for modes other thanthe normal merge mode. For example, it is also excellent that motioninformation such as MVs of blocks processed in affine mode in the pastmay be stored newest first, and may be used as MV candidates. The modeobtained by applying HMVP mode to affine mode may be referred to ashistory affine mode.

[MV Derivation>FRUC Mode]

Motion information may be derived at the decoder 200 side without beingsignaled from the encoder 100 side. For example, motion information maybe derived by performing motion estimation at the decoder 200 side. Atthis time, at the decoder 200 side, motion estimation is performedwithout using any pixel value in a current block. Modes in which motionestimation is performed at the decoder 200 side in this way include aframe rate up-conversion (FRUC) mode, a pattern matched motion vectorderivation (PMMVD) mode, etc.

One example of a FRUC process is illustrated in FIG. 43 . First, a listwhich indicates, as MV candidates, MVs for encoded blocks each of whichneighbors the current block spatially or temporally is generated byreferring to the MVs (the list may be an MV candidate list, and be alsoused as the MV candidate list for normal merge mode) (Step Si_1). Next,a best MV candidate is selected from the plurality of MV candidatesregistered in the MV candidate list (Step Si_2). For example, theevaluation values of the respective MV candidates included in the MVcandidate list are calculated, and one MV candidate is selected as thebest MV candidate based on the evaluation values. Based on the selectedbest MV candidate, a motion vector for the current block is then derived(Step Si_4). More specifically, for example, the selected best MVcandidate is directly derived as the MV for the current block. Inaddition, for example, the MV for the current block may be derived usingpattern matching in a surrounding region of a position which is includedin a reference picture and corresponds to the selected best MVcandidate. In other words, estimation using the pattern matching in areference picture and the evaluation values may be performed in thesurrounding region of the best MV candidate, and when there is an MVthat yields a better evaluation value, the best MV candidate may beupdated to the MV that yields the better evaluation value, and theupdated MV may be determined as the final MV for the current block.Update to the MV that yields the better evaluation value may not beperformed.

Lastly, inter predictor 126 generates a prediction image for the currentblock by performing motion compensation of the current block using thederived MV and the encoded reference picture (Step Si_5). The processesin Steps Si_1 to Si_5 are executed, for example, on each block. Forexample, when the processes in Steps Si_1 to Si_5 are executed on eachof all the blocks in the slice, inter prediction of the slice using theFRUC mode finishes. For example, when the processes in Steps Si_1 toSi_5 are executed on each of all the blocks in the picture, interprediction of the picture using the FRUC mode finishes. It is to benoted that not all the blocks included in the slice may be subjected tothe processes in Steps Si_1 to Si_5, and inter prediction of the sliceusing the FRUC mode may finish when part of the blocks are subjected tothe processes. Likewise, inter prediction of the picture using the FRUCmode may finish when the processes in Steps Si_1 to Si_5 are executed onpart of the blocks included in the picture.

Each sub-block may be processed similarly to the above-described case ofprocessing each block.

Evaluation values may be calculated according to various kinds ofmethods. For example, a comparison is made between a reconstructed imagein a region in a reference picture corresponding to an MV and areconstructed image in a determined region (the region may be, forexample, a region in another reference picture or a region in aneighboring block of a current picture, as indicated below). Thedifference between the pixel values of the two reconstructed images maybe used for an evaluation value of the MV. It is to be noted that anevaluation value may be calculated using information other than thevalue of the difference.

Next, pattern matching is described in detail. First, one MV candidateincluded in an MV candidate list (also referred to as a merge list) isselected as a starting point for estimation by pattern matching. As thepattern matching, either a first pattern matching or a second patternmatching may be used. The first pattern matching and the second patternmatching may be referred to as bilateral matching and template matching,respectively.

[MV Derivation>FRUC>Bilateral Matching]

In the first pattern matching, the pattern matching is performed betweentwo blocks which are located along a motion trajectory of a currentblock and included in two different reference pictures. Accordingly, inthe first pattern matching, a region in another reference picturelocated along the motion trajectory of the current block is used as adetermined region for calculating the evaluation value of theabove-described MV candidate.

FIG. 44 is a diagram for illustrating one example of the first patternmatching (bilateral matching) between the two blocks in the tworeference pictures located along the motion trajectory. As illustratedin FIG. 44 , in the first pattern matching, two motion vectors (MV0,MV1) are derived by estimating a pair which best matches among pairs oftwo blocks which are included in the two different reference pictures(Ref0, Ref1) and located along the motion trajectory of the currentblock (Cur block). More specifically, a difference between thereconstructed image at a specified position in the first encodedreference picture (Ref0) specified by an MV candidate and thereconstructed image at a specified position in the second encodedreference picture (Ref1) specified by a symmetrical MV obtained byscaling the MV candidate at a display time interval is derived for thecurrent block, and an evaluation value is calculated using the value ofthe obtained difference. It is excellent to select, as the best MV, theMV candidate which yields the best evaluation value among the pluralityof MV candidates.

In the assumption of a continuous motion trajectory, the motion vectors(MV0, MV1) specifying the two reference blocks are proportional totemporal distances (TD0, TD1) between the current picture (Cur Pic) andthe two reference pictures (Ref0, Ref1). For example, when the currentpicture is temporally located between the two reference pictures and thetemporal distances from the current picture to the respective tworeference pictures are equal to each other, mirror-symmetricalbi-directional MVs are derived in the first pattern matching.

[MV Derivation>FRUC>Template Matching]

In the second pattern matching (template matching), pattern matching isperformed between a block in a reference picture and a template in thecurrent picture (the template is a block neighboring the current blockin the current picture (the neighboring block is, for example, an upperand/or left neighboring block(s))). Accordingly, in the second patternmatching, the block neighboring the current block in the current pictureis used as the determined region for calculating the evaluation value ofthe above-described MV candidate.

FIG. 45 is a diagram for illustrating one example of pattern matching(template matching) between a template in a current picture and a blockin a reference picture. As illustrated in FIG. 45 , in the secondpattern matching, the MV for the current block (Cur block) is derived byestimating, in the reference picture (Ref0), the block which bestmatches the block neighboring the current block in the current picture(Cur Pic). More specifically, the difference between a reconstructedimage in an encoded region which neighbors both left and above or eitherleft or above and a reconstructed image which is in a correspondingregion in the encoded reference picture (Ref0) and is specified by an MVcandidate is derived, and an evaluation value is calculated using thevalue of the obtained difference. It is excellent to select, as the bestMV candidate, the MV candidate which yields the best evaluation valueamong the plurality of MV candidates.

Such information indicating whether to apply the FRUC mode (referred toas, for example, a FRUC flag) may be signaled at the CU level. Inaddition, when the FRUC mode is applied (for example, when a FRUC flagis true), information indicating an applicable pattern matching method(either the first pattern matching or the second pattern matching) maybe signaled at the CU level. It is to be noted that the signaling ofsuch information does not necessarily need to be performed at the CUlevel, and may be performed at another level (for example, at thesequence level, picture level, slice level, brick level, CTU level, orsub-block level).

[MV Derivation>Affine Mode]

The affine mode is a mode for generating an MV using affine transform.For example, an MV may be derived in units of a sub-block based onmotion vectors of a plurality of neighboring blocks. This mode is alsoreferred to as an affine motion compensation prediction mode.

FIG. 46A is a diagram for illustrating one example of MV derivation inunits of a sub-block based on MVs of a plurality of neighboring blocks.In FIG. 46A, the current block includes sixteen 4×4 pixel sub-blocks.Here, motion vector v₀ at an upper-left corner control point in thecurrent block is derived based on an MV of a neighboring block, andlikewise, motion vector v₁ at an upper-right corner control point in thecurrent block is derived based on an MV of a neighboring sub-block. Twomotion vectors v₀ and v₁ are projected according to an expression (1A)indicated below, and motion vectors (x_(x), v_(y)) for the respectivesub-blocks in the current block are derived.

$\begin{matrix}\left\lbrack {{Math}.1} \right\rbrack &  \\\left\{ \begin{matrix}{v_{x} = {{\frac{\left( {v_{1x} - v_{0x}} \right)}{w}x} - {\frac{\left( {v_{1y} - v_{0y}} \right)}{w}y} + v_{0x}}} \\{v_{y} = {{\frac{\left( {v_{1y} - v_{0y}} \right)}{w}x} + {\frac{\left( {v_{1x} - v_{0x}} \right)}{w}y} + v_{0y}}}\end{matrix} \right. & \left( {1A} \right)\end{matrix}$

Here, x and y indicate the horizontal position and the vertical positionof the sub-block, respectively, and w indicates a predeterminedweighting coefficient.

Such information indicating the affine mode (for example, referred to asan affine flag) may be signaled at the CU level. It is to be noted thatthe signaling of such information does not necessarily need to beperformed at the CU level, and may be performed at another level (forexample, at the sequence level, picture level, slice level, brick level,CTU level, or sub-block level).

In addition, the affine mode may include several modes for differentmethods for deriving MVs at the upper-left and upper-right cornercontrol points. For example, the affine modes include two modes whichare the affine inter mode (also referred to as an affine normal intermode) and the affine merge mode.

FIG. 46B is a diagram for illustrating one example of MV derivation inunits of a sub-block in affine mode in which three control points areused. In FIG. 46B, the current block includes, for example, sixteen 4×4pixel sub-blocks. Here, motion vector v₀ at an upper-left corner controlpoint in the current block is derived based on an MV of a neighboringblock. Here, motion vector v₁ at an upper-right corner control point inthe current block is derived based on an MV of a neighboring block, andlikewise, motion vector v₂ at a lower-left corner control point for thecurrent block is derived based on an MV of a neighboring block. Threemotion vectors v₀, v₁, and v₂ are projected according to an expression(1B) indicated below, and motion vectors (v_(x), v_(y)) for therespective sub-blocks in the current block are derived.

$\begin{matrix}\left\lbrack {{Math}.2} \right\rbrack &  \\\left\{ \begin{matrix}{v_{x} = {{\frac{\left( {v_{1x} - v_{0x}} \right)}{w}x} + {\frac{\left( {v_{2x} - v_{0x}} \right)}{h}y} + v_{0x}}} \\{v_{y} = {{\frac{\left( {v_{1y} - v_{0y}} \right)}{w}x} + {\frac{\left( {v_{2y} - v_{0y}} \right)}{h}y} + v_{0y}}}\end{matrix} \right. & \left( {1B} \right)\end{matrix}$

Here, x and y indicate the horizontal position and the vertical positionof the sub-block, respectively, and each of w and h indicates apredetermined weighting coefficient. Here, w may indicate the width of acurrent block, and h may indicate the height of the current block.

Affine modes in which different numbers of control points (for example,two and three control points) are used may be switched and signaled atthe CU level. It is to be noted that information indicating the numberof control points in affine mode used at the CU level may be signaled atanother level (for example, the sequence level, picture level, slicelevel, brick level, CTU level, or sub-block level).

In addition, such an affine mode in which three control points are usedmay include different methods for deriving MVs at the upper-left,upper-right, and lower-left corner control points. For example, theaffine modes in which three control points are used include two modeswhich are affine inter mode and affine merge mode, as in the case ofaffine modes in which two control points are used.

It is to be noted that, in the affine modes, the size of each sub-blockincluded in the current block may not be limited to 4×4 pixels, and maybe another size. For example, the size of each sub-block may be 8×8pixels.

[MV Derivation>Affine Mode>Control Point]

FIGS. 47A, 47B, and 47C are each a conceptual diagram for illustratingone example of MV derivation at control points in an affine mode.

As illustrated in FIG. 47A, in the affine mode, for example, MVpredictors at respective control points for a current block arecalculated based on a plurality of MVs corresponding to blocks encodedaccording to the affine mode among encoded block A (left), block B(upper), block C (upper-right), block D (lower-left), and block E(upper-left) which neighbor the current block. More specifically,encoded block A (left), block B (upper), block C (upper-right), block D(lower-left), and block E (upper-left) are checked in the listed order,and the first effective block encoded according to the affine mode isidentified. The MV at each control point for the current block iscalculated based on the plurality of MVs corresponding to the identifiedblock.

For example, as illustrated in FIG. 47B, when block A which neighbors tothe left of the current block has been encoded according to an affinemode in which two control points are used, motion vectors v₃ and v₄projected at the upper-left corner position and the upper-right cornerposition of the encoded block including block A are derived. Motionvector v₀ at the upper-left control point and motion vector v₁ at theupper-right control point for the current block are then calculated fromderived motion vectors v₃ and v₄.

For example, as illustrated in FIG. 47C, when block A which neighbors tothe left of the current block has been encoded according to an affinemode in which three control points are used, motion vectors v₃, v₄, andv₅ projected at the upper-left corner position, the upper-right cornerposition, and the lower-left corner position of the encoded blockincluding block A are derived. Motion vector v₀ at the upper-leftcontrol point for the current block, motion vector v₁ at the upper-rightcontrol point for the current block, and motion vector v₂ at thelower-left control point for the current block are then calculated fromderived motion vectors v₃, v₄, and v₅.

The MV derivation methods illustrated in FIGS. 47A to 47C may be used inthe MV derivation at each control point for the current block in StepSk_1 illustrated in FIG. 50 described later, or may be used for MVpredictor derivation at each control point for the current block in StepSj_1 illustrated in FIG. 51 described later.

FIGS. 48A and 48B are each a conceptual diagram for illustrating anotherexample of MV derivation at control points in affine mode.

FIG. 48A is a diagram for illustrating an affine mode in which twocontrol points are used.

In the affine mode, as illustrated in FIG. 48A, an MV selected from MVsat encoded block A, block B, and block C which neighbor the currentblock is used as motion vector v₀ at the upper-left corner control pointfor the current block. Likewise, an MV selected from MVs of encodedblock D and block E which neighbor the current block is used as motionvector v₁ at the upper-right corner control point for the current block.

FIG. 48B is a diagram for illustrating an affine mode in which threecontrol points are used.

In the affine mode, as illustrated in FIG. 48B, an MV selected from MVsat encoded block A, block B, and block C which neighbor the currentblock is used as motion vector v₀ at the upper-left corner control pointfor the current block. Likewise, an MV selected from MVs of encodedblock D and block E which neighbor the current block is used as motionvector v₁ at the upper-right corner control point for the current block.Furthermore, an MV selected from MVs of encoded block F and block Gwhich neighbor the current block is used as motion vector v₂ at thelower-left corner control point for the current block.

It is to be noted that the MV derivation methods illustrated in FIGS.48A and 48B may be used in the MV derivation at each control point forthe current block in Step Sk_1 illustrated in FIG. 50 described later,or may be used for MV predictor derivation at each control point for thecurrent block in Step Sj_1 illustrated in FIG. 51 described later.

Here, when affine modes in which different numbers of control points(for example, two and three control points) are used may be switched andsignaled at the CU level, the number of control points for an encodedblock and the number of control points for a current block may bedifferent from each other.

FIGS. 49A and 49B are each a conceptual diagram for illustrating oneexample of a method for MV derivation at control points when the numberof control points for an encoded block and the number of control pointsfor a current block are different from each other.

For example, as illustrated in FIG. 49A, a current block has threecontrol points at the upper-left corner, the upper-right corner, and thelower-left corner, and block A which neighbors to the left of thecurrent block has been encoded according to an affine mode in which twocontrol points are used. In this case, motion vectors v₃ and v₄projected at the upper-left corner position and the upper-right cornerposition in the encoded block including block A are derived. Motionvector v₀ at the upper-left corner control point and motion vector v₁ atthe upper-right corner control point for the current block are thencalculated from derived motion vectors v₃ and v₄. Furthermore, motionvector v₂ at the lower-left corner control point is calculated fromderived motion vectors v₀ and v₁.

For example, as illustrated in FIG. 49B, a current block has two controlpoints at the upper-left corner and the upper-right corner, and block Awhich neighbors to the left of the current block has been encodedaccording to an affine mode in which three control points are used. Inthis case, motion vectors v₃, v₄, and v₅ projected at the upper-leftcorner position in the encoded block including block A, the upper-rightcorner position in the encoded block, and the lower-left corner positionin the encoded block are derived. Motion vector v₀ at the upper-leftcorner control point for the current block and motion vector v₁ at theupper-right corner control point for the current block are thencalculated from derived motion vectors v₃, v₄, and v₅.

It is to be noted that the MV derivation methods illustrated in FIGS.49A and 49B may be used in the MV derivation at each control point forthe current block in Step Sk_1 illustrated in FIG. 50 described later,or may be used for MV predictor derivation at each control point for thecurrent block in Step Sj_1 illustrated in FIG. 51 described later.

[MV Derivation>Affine Mode>Affine Merge Mode]

FIG. 50 is a flow chart illustrating one example of the affine mergemode.

In the affine merge mode, first, inter predictor 126 derives MVs atrespective control points for a current block (Step Sk_1). The controlpoints are an upper-left corner point of the current block and anupper-right corner point of the current block as illustrated in FIG.46A, or an upper-left corner point of the current block, an upper-rightcorner point of the current block, and a lower-left corner point of thecurrent block as illustrated in FIG. 46B. At this time, inter predictor126 may encode MV selection information for identifying two or threederived MVs in a stream.

For example, when MV derivation methods illustrated in FIGS. 47A to 47Care used, as illustrated in FIG. 47A, inter predictor 126 checks encodedblock A (left), block B (upper), block C (upper-right), block D(lower-left), and block E (upper-left) in the listed order, andidentifies the first effective block encoded according to the affinemode.

Inter predictor 126 derives the MV at the control point using theidentified first effective block encoded according to the identifiedaffine mode. For example, when block A is identified and block A has twocontrol points, as illustrated in FIG. 47B, inter predictor 126calculates motion vector v₀ at the upper-left corner control point ofthe current block and motion vector v₁ at the upper-right corner controlpoint of the current block from motion vectors v₃ and v₄ at theupper-left corner of the encoded block including block A and theupper-right corner of the encoded block. For example, inter predictor126 calculates motion vector v₀ at the upper-left corner control pointof the current block and motion vector v₁ at the upper-right cornercontrol point of the current block by projecting motion vectors v₃ andv₄ at the upper-left corner and the upper-right corner of the encodedblock onto the current block.

Alternatively, when block A is identified and block A has three controlpoints, as illustrated in FIG. 47C, inter predictor 126 calculatesmotion vector v₀ at the upper-left corner control point of the currentblock, motion vector v₁ at the upper-right corner control point of thecurrent block, and motion vector v₂ at the lower-left corner controlpoint of the current block from motion vectors v₃, v₄, and v₅ at theupper-left corner of the encoded block including block A, theupper-right corner of the encoded block, and the lower-left corner ofthe encoded block. For example, inter predictor 126 calculates motionvector v₀ at the upper-left corner control point of the current block,motion vector v₁ at the upper-right corner control point of the currentblock, and motion vector v₂ at the lower-left corner control point ofthe current block by projecting motion vectors v₃, v₄, and v₅ at theupper-left corner, the upper-right corner, and the lower-left corner ofthe encoded block onto the current block.

It is to be noted that, as illustrated in FIG. 49A described above, MVsat three control points may be calculated when block A is identified andblock A has two control points, and that, as illustrated in FIG. 49Bdescribed above, MVs at two control points may be calculated when blockA is identified and block A has three control points.

Next, inter predictor 126 performs motion compensation of each of aplurality of sub-blocks included in the current block. In other words,inter predictor 126 calculates an MV for each of the plurality ofsub-blocks as an affine MV, using either two motion vectors v₀ and v₁and the above expression (1A) or three motion vectors v₀, v₁, and v₂ andthe above expression (1B) (Step Sk_2). Inter predictor 126 then performsmotion compensation of the sub-blocks using these affine MVs and encodedreference pictures (Step Sk_3). When the processes in Steps Sk_2 andSk_3 are executed for each of all the sub-blocks included in the currentblock, the process for generating a prediction image using the affinemerge mode for the current block finishes. In other words, motioncompensation of the current block is performed to generate a predictionimage of the current block.

It is to be noted that the above-described MV candidate list may begenerated in Step Sk_1. The MV candidate list may be, for example, alist including MV candidates derived using a plurality of MV derivationmethods for each control point. The plurality of MV derivation methodsmay be any combination of the MV derivation methods illustrated in FIGS.47A to 47C, the MV derivation methods illustrated in FIGS. 48A and 48B,the MV derivation methods illustrated in FIGS. 49A and 49B, and other MVderivation methods.

It is to be noted that MV candidate lists may include MV candidates in amode in which prediction is performed in units of a sub-block, otherthan the affine mode.

It is to be noted that, for example, an MV candidate list including MVcandidates in an affine merge mode in which two control points are usedand an affine merge mode in which three control points are used may begenerated as an MV candidate list. Alternatively, an MV candidate listincluding MV candidates in the affine merge mode in which two controlpoints are used and an MV candidate list including MV candidates in theaffine merge mode in which three control points are used may begenerated separately.

Alternatively, an MV candidate list including MV candidates in one ofthe affine merge mode in which two control points are used and theaffine merge mode in which three control points are used may begenerated. The MV candidate(s) may be, for example, MVs for encodedblock A (left), block B (upper), block C (upper-right), block D(lower-left), and block E (upper-left), or an MV for an effective blockamong the blocks.

It is to be noted that index indicating one of the MVs in an MVcandidate list may be transmitted as MV selection information.

[MV Derivation>Affine Mode>Affine Inter Mode]

FIG. 51 is a flow chart illustrating one example of an affine intermode.

In the affine inter mode, first, inter predictor 126 derives MVpredictors (v₀, v₁) or (v₀, v₁, v₂) of respective two or three controlpoints for a current block (Step Sj_1). The control points are anupper-left corner point for the current block, an upper-right cornerpoint of the current block, and a lower-left corner point for thecurrent block as illustrated in FIG. 46A or FIG. 46B.

For example, when the MV derivation methods illustrated in FIGS. 48A and48B are used, inter predictor 126 derives the MV predictors (v₀, v₁) or(v₀, v₁, v₂) at respective two or three control points for the currentblock by selecting MVs of any of the blocks among encoded blocks in thevicinity of the respective control points for the current blockillustrated in either FIG. 48A or FIG. 48B. At this time, interpredictor 126 encodes, in a stream, MV predictor selection informationfor identifying the selected two or three MV predictors.

For example, inter predictor 126 may determine, using a cost evaluationor the like, the block from which an MV as an MV predictor at a controlpoint is selected from among encoded blocks neighboring the currentblock, and may write, in a bitstream, a flag indicating which MVpredictor has been selected. In other words, inter predictor 126outputs, as a prediction parameter, the MV predictor selectioninformation such as a flag to entropy encoder 110 through predictionparameter generator 130.

Next, inter predictor 126 performs motion estimation (Steps Sj_3 andSj_4) while updating the MV predictor selected or derived in Step Sj_1(Step Sj_2). In other words, inter predictor 126 calculates, as anaffine MV, an MV of each of sub-blocks which corresponds to an updatedMV predictor, using either the expression (1A) or expression (1B)described above (Step Sj_3). Inter predictor 126 then performs motioncompensation of the sub-blocks using these affine MVs and encodedreference pictures (Step Sj_4). The processes in Steps Sj_3 and Sj_4 areexecuted on all the blocks in the current block each time an MVpredictor is updated in Step Sj_2. As a result, for example, interpredictor 126 determines the MV predictor which yields the smallest costas the MV at a control point in a motion estimation loop (Step Sj_5). Atthis time, inter predictor 126 further encodes, in the stream, thedifference value between the determined MV and the MV predictor as an MVdifference. In other words, inter predictor 126 outputs the MVdifference as a prediction parameter to entropy encoder 110 throughprediction parameter generator 130.

Lastly, inter predictor 126 generates a prediction image for the currentblock by performing motion compensation of the current block using thedetermined MV and the encoded reference picture (Step Sj_6).

It is to be noted that the above-described MV candidate list may begenerated in Step Sj_1. The MV candidate list may be, for example, alist including MV candidates derived using a plurality of MV derivationmethods for each control point. The plurality of MV derivation methodsmay be any combination of the MV derivation methods illustrated in FIGS.47A to 47C, the MV derivation methods illustrated in FIGS. 48A and 48B,the MV derivation methods illustrated in FIGS. 49A and 49B, and other MVderivation methods.

It is to be noted that the MV candidate list may include MV candidatesin a mode in which prediction is performed in units of a sub-block,other than the affine mode.

It is to be noted that, for example, an MV candidate list including MVcandidates in an affine inter mode in which two control points are usedand an affine inter mode in which three control points are used may begenerated as an MV candidate list. Alternatively, an MV candidate listincluding MV candidates in the affine inter mode in which two controlpoints are used and an MV candidate list including MV candidates in theaffine inter mode in which three control points are used may begenerated separately. Alternatively, an MV candidate list including MVcandidates in one of the affine inter mode in which two control pointsare used and the affine inter mode in which three control points areused may be generated. The MV candidate(s) may be, for example, MVs forencoded block A (left), block B (upper), block C (upper-right), block D(lower-left), and block E (upper-left), or an MV for an effective blockamong the blocks.

It is to be noted that index indicating one of the MV candidates in anMV candidate list may be transmitted as MV predictor selectioninformation.

[MV Derivation>Triangle Mode]

Inter predictor 126 generates one rectangular prediction image for arectangular current block in the above example. However, inter predictor126 may generate a plurality of prediction images each having a shapedifferent from a rectangle for the rectangular current block, and maycombine the plurality of prediction images to generate the finalrectangular prediction image. The shape different from a rectangle maybe, for example, a triangle.

FIG. 52A is a diagram for illustrating generation of two triangularprediction images.

Inter predictor 126 generates a triangular prediction image byperforming motion compensation of a first partition having a triangularshape in a current block by using a first MV of the first partition, togenerate a triangular prediction image. Likewise, inter predictor 126generates a triangular prediction image by performing motioncompensation of a second partition having a triangular shape in acurrent block by using a second MV of the second partition, to generatea triangular prediction image. Inter predictor 126 then generates aprediction image having the same rectangular shape as the rectangularshape of the current block by combining these prediction images.

It is to be noted that a first prediction image having a rectangularshape corresponding to a current block may be generated as a predictionimage for a first partition, using a first MV. In addition, a secondprediction image having a rectangular shape corresponding to a currentblock may be generated as a prediction image for a second partition,using a second MV. A prediction image for the current block may begenerated by performing a weighted addition of the first predictionimage and the second prediction image. It is to be noted that the partwhich is subjected to the weighted addition may be a partial regionacross the boundary between the first partition and the secondpartition.

FIG. 52B is a conceptual diagram for illustrating examples of a firstportion of a first partition which overlaps with a second partition, andfirst and second sets of samples which may be weighted as part of acorrection process. The first portion may be, for example, one fourth ofthe width or height of the first partition. In another example, thefirst portion may have a width corresponding to N samples adjacent to anedge of the first partition, where N is an integer greater than zero,and N may be, for example, the integer 2. As illustrated, the leftexample of FIG. 52B shows a rectangular partition having a rectangularportion with a width which is one fourth of the width of the firstpartition, with the first set of samples including samples outside ofthe first portion and samples inside of the first portion, and thesecond set of samples including samples within the first portion. Thecenter example of FIG. 52B shows a rectangular partition having arectangular portion with a height which is one fourth of the height ofthe first partition, with the first set of samples including samplesoutside of the first portion and samples inside of the first portion,and the second set of samples including samples within the firstportion. The right example of FIG. 52B shows a triangular partitionhaving a polygonal portion with a height which corresponds to twosamples, with the first set of samples including samples outside of thefirst portion and samples inside of the first portion, and the secondset of samples including samples within the first portion.

The first portion may be a portion of the first partition which overlapswith an adjacent partition. FIG. 52C is a conceptual diagram forillustrating a first portion of a first partition, which is a portion ofthe first partition that overlaps with a portion of an adjacentpartition. For ease of illustration, a rectangular partition having anoverlapping portion with a spatially adjacent rectangular partition isshown. Partitions having other shapes, such as triangular partitions,may be employed, and the overlapping portions may overlap with aspatially or temporally adjacent partition.

In addition, although an example is given in which a prediction image isgenerated for each of two partitions using inter prediction, aprediction image may be generated for at least one partition using intraprediction.

FIG. 53 is a flow chart illustrating one example of a triangle mode.

In the triangle mode, first, inter predictor 126 splits the currentblock into the first partition and the second partition (Step Sx_1). Atthis time, inter predictor 126 may encode, in a stream, partitioninformation which is information related to the splitting into thepartitions as a prediction parameter. In other words, inter predictor126 may output the partition information as the prediction parameter toentropy encoder 110 through prediction parameter generator 130.

First, inter predictor 126 obtains a plurality of MV candidates for acurrent block based on information such as MVs of a plurality of encodedblocks temporally or spatially surrounding the current block (StepSx_2). In other words, inter predictor 126 generates an MV candidatelist.

Inter predictor 126 then selects the MV candidate for the firstpartition and the MV candidate for the second partition as a first MVand a second MV, respectively, from the plurality of MV candidatesobtained in Step Sx_2 (Step Sx_3). At this time, inter predictor 126encodes, in a stream, MV selection information for identifying theselected MV candidate, as a prediction parameter. In other words, interpredictor 126 outputs the MV selection information as a predictionparameter to entropy encoder 110 through prediction parameter generator130.

Next, inter predictor 126 generates a first prediction image byperforming motion compensation using the selected first MV and anencoded reference picture (Step Sx_4). Likewise, inter predictor 126generates a second prediction image by performing motion compensationusing the selected second MV and an encoded reference picture (StepSx_5).

Lastly, inter predictor 126 generates a prediction image for the currentblock by performing a weighted addition of the first prediction imageand the second prediction image (Step Sx_6).

It is to be noted that, although the first partition and the secondpartition are triangles in the example illustrated in FIG. 52A, thefirst partition and the second partition may be trapezoids, or othershapes different from each other. Furthermore, although the currentblock includes two partitions in the example illustrated in FIG. 52A,the current block may include three or more partitions.

In addition, the first partition and the second partition may overlapwith each other. In other words, the first partition and the secondpartition may include the same pixel region. In this case, a predictionimage for a current block may be generated using a prediction image inthe first partition and a prediction image in the second partition.

In addition, although the example in which the prediction image isgenerated for each of the two partitions using inter prediction has beenillustrated, a prediction image may be generated for at least onepartition using intra prediction.

It is to be noted that the MV candidate list for selecting the first MVand the MV candidate list for selecting the second MV may be differentfrom each other, or the MV candidate list for selecting the first MV maybe also used as the MV candidate list for selecting the second MV.

It is to be noted that partition information may include an indexindicating the splitting direction in which at least a current block issplit into a plurality of partitions. The MV selection information mayinclude an index indicating the selected first MV and an indexindicating the selected second MV. One index may indicate a plurality ofpieces of information. For example, one index collectively indicating apart or the entirety of partition information and a part or the entiretyof MV selection information may be encoded.

[MV Derivation>ATMVP Mode]

FIG. 54 is a diagram illustrating one example of an ATMVP mode in whichan MV is derived in units of a sub-block.

The ATMVP mode is a mode categorized into the merge mode. For example,in the ATMVP mode, an MV candidate for each sub-block is registered inan MV candidate list for use in normal merge mode.

More specifically, in the ATMVP mode, first, as illustrated in FIG. 54 ,a temporal MV reference block associated with a current block isidentified in an encoded reference picture specified by an MV (MV0) of aneighboring block located at the lower-left position with respect to thecurrent block. Next, in each sub-block in the current block, the MV usedto encode the region corresponding to the sub-block in the temporal MVreference block is identified. The MV identified in this way is includedin an MV candidate list as an MV candidate for the sub-block in thecurrent block. When the MV candidate for each sub-block is selected fromthe MV candidate list, the sub-block is subjected to motion compensationin which the MV candidate is used as the MV for the sub-block. In thisway, a prediction image for each sub-block is generated.

Although the block located at the lower-left position with respect tothe current block is used as a surrounding MV reference block in theexample illustrated in FIG. 54 , it is to be noted that another blockmay be used. In addition, the size of the sub-block may be 4×4 pixels,8×8 pixels, or another size. The size of the sub-block may be switchedfor a unit such as a slice, brick, picture, etc.

[Motion Estimation>DMVR]

FIG. 55 is a diagram illustrating a relationship between a merge modeand DMVR.

Inter predictor 126 derives an MV for a current block according to themerge mode (Step Sl_1). Next, inter predictor 126 determines whether toperform estimation of an MV that is motion estimation (Step Sl_2). Here,when determining not to perform motion estimation (No in Step Sl_2),inter predictor 126 determines the MV derived in Step Sl_1 as the finalMV for the current block (Step Sl_4). In other words, in this case, theMV for the current block is determined according to the merge mode.

When determining to perform motion estimation in Step Sl_1 (Yes in StepSl_2), inter predictor 126 derives the final MV for the current block byestimating a surrounding region of the reference picture specified bythe MV derived in Step Sl_1 (Step Sl_3). In other words, in this case,the MV for the current block is determined according to the DMVR.

FIG. 56 is a conceptual diagram for illustrating another example of DMVRfor determining an MV.

First, in the merge mode for example, MV candidates (L0 and L1) areselected for the current block. A reference pixel is identified from afirst reference picture (L0) which is an encoded picture in the L0 listaccording to the MV candidate (L0). Likewise, a reference pixel isidentified from a second reference picture (L1) which is an encodedpicture in the L1 list according to the MV candidate (L1). A template isgenerated by calculating an average of these reference pixels.

Next, each of the surrounding regions of MV candidates of the firstreference picture (L0) and the second reference picture (L1) areestimated using the template, and the MV which yields the smallest costis determined to be the final MV. It is to be noted that the cost may becalculated, for example, using a difference value between each of thepixel values in the template and a corresponding one of the pixel valuesin the estimation region, the values of MV candidates, etc.

Exactly the same processes described here do not always need to beperformed. Any process for enabling derivation of the final MV byestimation in surrounding regions of MV candidates may be used.

FIG. 57 is a conceptual diagram for illustrating another example of DMVRfor determining an MV. Unlike the example of DMVR illustrated in FIG. 56, in the example illustrated in FIG. 57 , costs are calculated withoutgenerating any template.

First, inter predictor 126 estimates a surrounding region of a referenceblock included in each of reference pictures in the L0 list and L1 list,based on an initial MV which is an MV candidate obtained from each MVcandidate list. For example, as illustrated in FIG. 57 , the initial MVcorresponding to the reference block in the L0 list is InitMV_L0, andthe initial MV corresponding to the reference block in the L1 list isInitMV_L1. In motion estimation, inter predictor 126 firstly sets asearch position for the reference picture in the L0 list. Based on theposition indicated by the vector difference indicating the searchposition to be set, specifically, the initial MV (that is, InitMV_L0),the vector difference to the search position is MVd_L0. Inter predictor126 then determines the estimation position in the reference picture inthe L1 list. This search position is indicated by the vector differenceto the search position from the position indicated by the initial MV(that is, InitMV_L1). More specifically, inter predictor 126 determinesthe vector difference as MVd_L1 by mirroring of MVd_L0. In other words,inter predictor 126 determines the position which is symmetrical withrespect to the position indicated by the initial MV to be the searchposition in each reference picture in the L0 list and the L1 list. Interpredictor 126 calculates, for each search position, the total sum of theabsolute differences (SADs) between values of pixels at search positionsin blocks as a cost, and finds out the search position that yields thesmallest cost.

FIG. 58A is a diagram illustrating one example of motion estimation inDMVR, and FIG. 58B is a flow chart illustrating one example of themotion estimation.

First, in Step 1, inter predictor 126 calculates the cost between thesearch position (also referred to as a starting point) indicated by theinitial MV and eight surrounding search positions. Inter predictor 126then determines whether the cost at each of the search positions otherthan the starting point is the smallest. Here, when determining that thecost at the search position other than the starting point is thesmallest, inter predictor 126 changes a target to the search position atwhich the smallest cost is obtained, and performs the process in Step 2.When the cost at the starting point is the smallest, inter predictor 126skips the process in Step 2 and performs the process in Step 3.

In Step 2, inter predictor 126 performs the search similar to theprocess in Step 1, regarding, as a new starting point, the searchposition after the target change according to the result of the processin Step 1. Inter predictor 126 then determines whether the cost at eachof the search positions other than the starting point is the smallest.Here, when determining that the cost at the search position other thanthe starting point is the smallest, inter predictor 126 performs theprocess in Step 4. When the cost at the starting point is the smallest,inter predictor 126 performs the process in Step 3.

In Step 4, inter predictor 126 regards the search position at thestarting point as the final search position, and determines thedifference between the position indicated by the initial MV and thefinal search position to be a vector difference.

In Step 3, inter predictor 126 determines the pixel position atsub-pixel accuracy at which the smallest cost is obtained, based on thecosts at the four points located at upper, lower, left, and rightpositions with respect to the starting point in Step 1 or Step 2, andregards the pixel position as the final search position. The pixelposition at the sub-pixel accuracy is determined by performing weightedaddition of each of the four upper, lower, left, and right vectors ((0,1), (0, −1), (−1, 0), and (1, 0)), using, as a weight, the cost at acorresponding one of the four search positions. Inter predictor 126 thendetermines the difference between the position indicated by the initialMV and the final search position to be the vector difference.

[Motion Compensation>BIO/OBMC/LIC]

Motion compensation involves a mode for generating a prediction image,and correcting the prediction image. The mode is, for example, BIO,OBMC, and LIC to be described later.

FIG. 59 is a flow chart illustrating one example of generation of aprediction image.

Inter predictor 126 generates a prediction image (Step Sm_1), andcorrects the prediction image according to any of the modes describedabove (Step Sm_2).

FIG. 60 is a flow chart illustrating another example of generation of aprediction image.

Inter predictor 126 derives an MV of a current block (Step Sn_1). Next,inter predictor 126 generates a prediction image using the MV (StepSn_2), and determines whether to perform a correction process (StepSn_3). Here, when determining to perform a correction process (Yes inStep Sn_3), inter predictor 126 generates the final prediction image bycorrecting the prediction image (Step Sn_4). It is to be noted that, inLIC described later, luminance and chrominance may be corrected in StepSn_4. When determining not to perform a correction process (No in StepSn_3), inter predictor 126 outputs the prediction image as the finalprediction image without correcting the prediction image (Step Sn_5).

[Motion Compensation>OBMC]

It is to be noted that an inter prediction image may be generated usingmotion information for a neighboring block in addition to motioninformation for the current block obtained by motion estimation. Morespecifically, an inter prediction image may be generated for eachsub-block in a current block by performing weighted addition of aprediction image based on the motion information obtained by motionestimation (in a reference picture) and a prediction image based on themotion information of the neighboring block (in the current picture).Such inter prediction (motion compensation) is also referred to asoverlapped block motion compensation (OBMC) or an OBMC mode.

In OBMC mode, information indicating a sub-block size for OBMC (referredto as, for example, an OBMC block size) may be signaled at the sequencelevel. Moreover, information indicating whether to apply the OBMC mode(referred to as, for example, an OBMC flag) may be signaled at the CUlevel. It is to be noted that the signaling of such information does notnecessarily need to be performed at the sequence level and CU level, andmay be performed at another level (for example, at the picture level,slice level, brick level, CTU level, or sub-block level).

The OBMC mode will be described in further detail. FIGS. 61 and 62 are aflow chart and a conceptual diagram for illustrating an outline of aprediction image correction process performed by OBMC.

First, as illustrated in FIG. 62 , a prediction image (Pred) by normalmotion compensation is obtained using an MV assigned to a current block.In FIG. 62 , the arrow “MV” points a reference picture, and indicateswhat the current block of the current picture refers to in order toobtain the prediction image.

Next, a prediction image (Pred_L) is obtained by applying a motionvector (MV_L) which has been already derived for the encoded blockneighboring to the left of the current block to the current block(re-using the motion vector for the current block). The motion vector(MV_L) is indicated by an arrow “MV_L” indicating a reference picturefrom a current block. A first correction of a prediction image isperformed by overlapping two prediction images Pred and Pred_L. Thisprovides an effect of blending the boundary between neighboring blocks.

Likewise, a prediction image (Pred_U) is obtained by applying an MV(MV_U) which has been already derived for the encoded block neighboringabove the current block to the current block (re-using the MV for thecurrent block). The MV (MV_U) is indicated by an arrow “MV_U” indicatinga reference picture from a current block. A second correction of aprediction image is performed by overlapping the prediction image Pred_Uto the prediction images (for example, Pred and Pred_L) on which thefirst correction has been performed. This provides an effect of blendingthe boundary between neighboring blocks. The prediction image obtainedby the second correction is the one in which the boundary between theneighboring blocks has been blended (smoothed), and thus is the finalprediction image of the current block.

Although the above example is a two-path correction method using leftand upper neighboring blocks, it is to be noted that the correctionmethod may be three- or more-path correction method using also the rightneighboring block and/or the lower neighboring block.

It is to be noted that the region in which such overlapping is performedmay be only part of a region near a block boundary instead of the pixelregion of the entire block.

It is to be noted that the prediction image correction process accordingto OBMC for obtaining one prediction image Pred from one referencepicture by overlapping additional prediction images Pred_L and Pred_Uhas been described above. However, when a prediction image is correctedbased on a plurality of reference images, a similar process may beapplied to each of the plurality of reference pictures. In such a case,after corrected prediction images are obtained from the respectivereference pictures by performing OBMC image correction based on theplurality of reference pictures, the obtained corrected predictionimages are further overlapped to obtain the final prediction image.

It is to be noted that, in OBMC, a current block unit may be a PU or asub-block unit obtained by further splitting the PU.

One example of a method for determining whether to apply OBMC is amethod for using an obmc_flag which is a signal indicating whether toapply OBMC. As one specific example, encoder 100 may determine whetherthe current block belongs to a region having complicated motion. Encoder100 sets the obmc_flag to a value of “1” when the block belongs to aregion having complicated motion and applies OBMC when encoding, andsets the obmc_flag to a value of “0” when the block does not belong to aregion having complicated motion and encodes the block without applyingOBMC. Decoder 200 switches between application and non-application ofOBMC by decoding the obmc_flag written in a stream.

[Motion Compensation>BIO]

Next, an MV derivation method is described. First, a mode for derivingan MV based on a model assuming uniform linear motion is described. Thismode is also referred to as a bi-directional optical flow (BIO) mode. Inaddition, this bi-directional optical flow may be written as BDOFinstead of BIO.

FIG. 63 is a diagram for illustrating a model assuming uniform linearmotion. In FIG. 63 , (v_(x), v_(y)) indicates a velocity vector, and τ0and τ1 indicate temporal distances between a current picture (Cur Pic)and two reference pictures (Ref₀, Ref₁). (MVx₀, MVy₀) indicates an MVcorresponding to reference picture Ref₀, and (MVx₁, MVy₁) indicates anMV corresponding to reference picture Ref₁.

Here, under the assumption of uniform linear motion exhibited by avelocity vector (v_(x), v_(y)), (MVx₀, MVy₀) and (MVx₁, MVy₁) arerepresented as (vxτ0, vyτ0) and (−vxτ1, −vyτ1), respectively, and thefollowing optical flow equation (2) is given.[MATH. 3]∂I ^((k)) /∂t+v _(x) ∂I ^((k)) /∂x+v _(y) ∂I ^((k)) /∂y=0  (2)

Here, I(k) denotes a luma value from reference image k (k=0, 1) aftermotion compensation. This optical flow equation shows that the sum of(i) the time derivative of the luma value, (ii) the product of thehorizontal velocity and the horizontal component of the spatial gradientof a reference image, and (iii) the product of the vertical velocity andthe vertical component of the spatial gradient of a reference image isequal to zero. A motion vector of each block obtained from, for example,an MV candidate list may be corrected in units of a pixel, based on acombination of the optical flow equation and Hermite interpolation.

It is to be noted that a motion vector may be derived on the decoder 200side using a method other than deriving a motion vector based on a modelassuming uniform linear motion. For example, a motion vector may bederived in units of a sub-block based on MVs of a plurality ofneighboring blocks.

FIG. 64 is a flow chart illustrating one example of inter predictionaccording to BIO. FIG. 65 is a diagram illustrating one example of aconfiguration of inter predictor 126 which performs inter predictionaccording to BIO.

As illustrated in FIG. 65 , inter predictor 126 includes, for example,memory 126 a, interpolated image deriver 126 b, gradient image deriver126 c, optical flow deriver 126 d, correction value deriver 126 e, andprediction image corrector 126 f. It is to be noted that memory 126 amay be frame memory 122.

Inter predictor 126 derives two motion vectors (M0, M1), using tworeference pictures (Ref₀, Ref₁) different from the picture (Cur Pic)including a current block. Inter predictor 126 then derives a predictionimage for the current block using the two motion vectors (M0, M1) (StepSy_1). It is to be noted that motion vector M0 is motion vector (MVx₀,MVy₀) corresponding to reference picture Ref₀, and motion vector M1 ismotion vector (MVx₁, MVy₁) corresponding to reference picture Ref₁.

Next, interpolated image deriver 126 b derives interpolated image I⁰ forthe current block, using motion vector M0 and reference picture L0 byreferring to memory 126 a. Next, interpolated image deriver 126 bderives interpolated image I¹ for the current block, using motion vectorM1 and reference picture L1 by referring to memory 126 a (Step Sy_2).Here, interpolated image I⁰ is an image included in reference pictureRef₀ and to be derived for the current block, and interpolated image I¹is an image included in reference picture Ref₁ and to be derived for thecurrent block. Each of interpolated image I0 and interpolated image I¹may be the same in size as the current block. Alternatively, each ofinterpolated image I⁰ and interpolated image I¹ may be an image largerthan the current block. Furthermore, interpolated image I0 andinterpolated image I¹ may include a prediction image obtained by usingmotion vectors (M0, M1) and reference pictures (L0, L1) and applying amotion compensation filter.

In addition, gradient image deriver 126 c derives gradient images (Ix⁰,Ix¹, Iy⁰, Iy¹) of the current block, from interpolated image I0 andinterpolated image I¹. It is to be noted that the gradient images in thehorizontal direction are (Ix⁰, Ix¹), and the gradient images in thevertical direction are (Iy⁰, Iy¹). Gradient image deriver 126 c mayderive each gradient image by, for example, applying a gradient filterto the interpolated images. It is only necessary that a gradient imageindicate the amount of spatial change in pixel value along thehorizontal direction or the vertical direction.

Next, optical flow deriver 126 d derives, for each sub-block of thecurrent block, an optical flow (vx, vy) which is a velocity vector,using the interpolated images (I⁰, I¹) and the gradient images (Ix⁰,Ix¹, Iy⁰, Iy¹). The optical flow indicates coefficients for correctingthe amount of spatial pixel movement, and may be referred to as a localmotion estimation value, a corrected motion vector, or a correctedweighting vector. As one example, a sub-block may be 4×4 pixel sub-CU.It is to be noted that the optical flow derivation may be performed foreach pixel unit, or the like, instead of being performed for eachsub-block.

Next, inter predictor 126 corrects a prediction image for the currentblock using the optical flow (vx, vy). For example, correction valuederiver 126 e derives a correction value for the value of a pixelincluded in a current block, using the optical flow (vx, vy) (StepSy_5). Prediction image corrector 126 f may then correct the predictionimage for the current block using the correction value (Step Sy_6). Itis to be noted that the correction value may be derived in units of apixel, or may be derived in units of a plurality of pixels or in unitsof a sub-block.

It is to be noted that the BIO process flow is not limited to theprocess disclosed in FIG. 64 . Only part of the processes disclosed inFIG. 64 may be performed, or a different process may be added or used asa replacement, or the processes may be executed in a differentprocessing order.

[Motion Compensation>LIC]

Next, one example of a mode for generating a prediction image(prediction) using a local illumination compensation (LIC) is described.

FIG. 66A is a diagram for illustrating one example of a prediction imagegeneration method using a luminance correction process performed by LIC.

FIG. 66B is a flow chart illustrating one example of a prediction imagegeneration method using the LIC.

First, inter predictor 126 derives an MV from an encoded referencepicture, and obtains a reference image corresponding to the currentblock (Step Sz_1).

Next, inter predictor 126 extracts, for the current block, informationindicating how the luma value has changed between the current block andthe reference picture (Step Sz_2). This extraction is performed based onthe luma pixel values of the encoded left neighboring reference region(surrounding reference region) and the encoded upper neighboringreference region (surrounding reference region) in the current picture,and the luma pixel values at the corresponding positions in thereference picture specified by the derived MVs. Inter predictor 126calculates a luminance correction parameter, using the informationindicating how the luma value has changed (Step Sz_3).

Inter predictor 126 generates a prediction image for the current blockby performing a luminance correction process in which the luminancecorrection parameter is applied to the reference image in the referencepicture specified by the MV (Step Sz_4). In other words, the predictionimage which is the reference image in the reference picture specified bythe MV is subjected to the correction based on the luminance correctionparameter. In this correction, luminance may be corrected, orchrominance may be corrected. In other words, a chrominance correctionparameter may be calculated using information indicating how chrominancehas changed, and a chrominance correction process may be performed.

It is to be noted that the shape of the surrounding reference regionillustrated in FIG. 66A is one example; another shape may be used.

Moreover, although the process in which a prediction image is generatedfrom a single reference picture has been described here, cases in whicha prediction image is generated from a plurality of reference picturescan be described in the same manner. The prediction image may begenerated after performing a luminance correction process of thereference images obtained from the reference pictures in the same manneras described above.

One example of a method for determining whether to apply LIC is a methodfor using a lic_flag which is a signal indicating whether to apply theLIC. As one specific example, encoder 100 determines whether the currentblock belongs to a region having a luminance change. Encoder 100 setsthe lic_flag to a value of “1” when the block belongs to a region havinga luminance change and applies LIC when encoding, and sets the lic_flagto a value of “0” when the block does not belong to a region having aluminance change and performs encoding without applying LIC. Decoder 200may decode the lic_flag written in the stream and decode the currentblock by switching between application and non-application of LIC inaccordance with the flag value.

One example of a different method of determining whether to apply a LICprocess is a determining method in accordance with whether a LIC processhas been applied to a surrounding block. As one specific example, when acurrent block has been processed in merge mode, inter predictor 126determines whether an encoded surrounding block selected in MVderivation in merge mode has been encoded using LIC. Inter predictor 126performs encoding by switching between application and non-applicationof LIC according to the result. It is to be noted that, also in thisexample, the same processes are applied to processes at the decoder 200side.

The luminance correction (LIC) process has been described with referenceto FIGS. 66A and 66B, and is further described below.

First, inter predictor 126 derives an MV for obtaining a reference imagecorresponding to a current block from a reference picture which is anencoded picture.

Next, inter predictor 126 extracts information indicating how the lumavalue of the reference picture has been changed to the luma value of thecurrent picture, using the luma pixel values of encoded surroundingreference regions which neighbor to the left of and above the currentblock and the luma pixel values in the corresponding positions in thereference pictures specified by MVs, and calculates a luminancecorrection parameter. For example, it is assumed that the luma pixelvalue of a given pixel in the surrounding reference region in thecurrent picture is p0, and that the luma pixel value of the pixelcorresponding to the given pixel in the surrounding reference region inthe reference picture is p1. Inter predictor 126 calculates coefficientsA and B for optimizing A×p1+B=p0 as the luminance correction parameterfor a plurality of pixels in the surrounding reference region.

Next, inter predictor 126 performs a luminance correction process usingthe luminance correction parameter for the reference image in thereference picture specified by the MV, to generate a prediction imagefor the current block. For example, it is assumed that the luma pixelvalue in the reference image is p2, and that the luminance-correctedluma pixel value of the prediction image is p3. Inter predictor 126generates the prediction image after being subjected to the luminancecorrection process by calculating A×p2+B=p3 for each of the pixels inthe reference image.

For example, a region having a determined number of pixels extractedfrom each of an upper neighboring pixel and a left neighboring pixel maybe used as a surrounding reference region. In addition, the surroundingreference region is not limited to a region which neighbors the currentblock, and may be a region which does not neighbor the current block. Inthe example illustrated in FIG. 66A, the surrounding reference region inthe reference picture may be a region specified by another MV in acurrent picture, from a surrounding reference region in the currentpicture. For example, the other MV may be an MV in a surroundingreference region in the current picture.

Although operations performed by encoder 100 have been described here,it is to be noted that decoder 200 performs similar operations.

It is to be noted that LIC may be applied not only to luma but also tochroma. At this time, a correction parameter may be derived individuallyfor each of Y, Cb, and Cr, or a common correction parameter may be usedfor any of Y, Cb, and Cr.

In addition, the LIC process may be applied in units of a sub-block. Forexample, a correction parameter may be derived using a surroundingreference region in a current sub-block and a surrounding referenceregion in a reference sub-block in a reference picture specified by anMV of the current sub-block.

[Prediction Controller]

Prediction controller 128 selects one of an intra prediction image (animage or a signal output from intra predictor 124) and an interprediction image (an image or a signal output from inter predictor 126),and outputs the selected prediction image to subtractor 104 and adder116.

[Prediction Parameter Generator]

Prediction parameter generator 130 may output information related tointra prediction, inter prediction, selection of a prediction image inprediction controller 128, etc. as a prediction parameter to entropyencoder 110. Entropy encoder 110 may generate a stream, based on theprediction parameter which is input from prediction parameter generator130 and quantized coefficients which are input from quantizer 108. Theprediction parameter may be used in decoder 200. Decoder 200 may receiveand decode the stream, and perform the same processes as the predictionprocesses performed by intra predictor 124, inter predictor 126, andprediction controller 128. The prediction parameter may include (i) aselection prediction signal (for example, an MV, a prediction type, or aprediction mode used by intra predictor 124 or inter predictor 126), or(ii) an optional index, a flag, or a value which is based on aprediction process performed in each of intra predictor 124, interpredictor 126, and prediction controller 128, or which indicates theprediction process.

[Decoder]

Next, decoder 200 capable of decoding a stream output from encoder 100described above is described. FIG. 67 is a block diagram illustrating aconfiguration of decoder 200 according to this embodiment. Decoder 200is an apparatus which decodes a stream that is an encoded image in unitsof a block.

As illustrated in FIG. 67 , decoder 200 includes entropy decoder 202,inverse quantizer 204, inverse transformer 206, adder 208, block memory210, loop filter 212, frame memory 214, intra predictor 216, interpredictor 218, prediction controller 220, prediction parameter generator222, and splitting determiner 224. It is to be noted that intrapredictor 216 and inter predictor 218 are configured as part of aprediction executor.

[Mounting Example of Decoder]

FIG. 68 is a block diagram illustrating a mounting example of decoder200. Decoder 200 includes processor b1 and memory b2. For example, theplurality of constituent elements of decoder 200 illustrated in FIG. 67are mounted on processor b1 and memory b2 illustrated in FIG. 68 .

Processor b1 is circuitry which performs information processing and isaccessible to memory b2. For example, processor b1 is a dedicated orgeneral electronic circuit which decodes a stream. Processor b1 may be aprocessor such as a CPU. In addition, processor b1 may be an aggregateof a plurality of electronic circuits. In addition, for example,processor b1 may take the roles of two or more constituent elementsother than a constituent element for storing information out of theplurality of constituent elements of decoder 200 illustrated in FIG. 67, etc.

Memory b2 is dedicated or general memory for storing information that isused by processor b1 to decode a stream. Memory b2 may be electroniccircuitry, and may be connected to processor b1. In addition, memory b2may be included in processor b1. In addition, memory b2 may be anaggregate of a plurality of electronic circuits. In addition, memory b2may be a magnetic disc, an optical disc, or the like, or may berepresented as a storage, a medium, or the like. In addition, memory b2may be non-volatile memory, or volatile memory.

For example, memory b2 may store an image or a stream. In addition,memory b2 may store a program for causing processor b1 to decode astream.

In addition, for example, memory b2 may take the roles of two or moreconstituent elements for storing information out of the plurality ofconstituent elements of decoder 200 illustrated in FIG. 67 , etc. Morespecifically, memory b2 may take the roles of block memory 210 and framememory 214 illustrated in FIG. 67 . More specifically, memory b2 maystore a reconstructed image (specifically, a reconstructed block, areconstructed picture, or the like).

It is to be noted that, in decoder 200, not all of the plurality ofconstituent elements illustrated in FIG. 67 , etc. may be implemented,and not all the processes described above may be performed. Part of theconstituent elements indicated in FIG. 67 , etc. may be included inanother device, or part of the processes described above may beperformed by another device.

Hereinafter, an overall flow of the processes performed by decoder 200is described, and then each of the constituent elements included indecoder 200 is described. It is to be noted that, some of theconstituent elements included in decoder 200 perform the same processesas performed by some of the constituent elements included in encoder100, and thus the same processes are not repeatedly described in detail.For example, inverse quantizer 204, inverse transformer 206, adder 208,block memory 210, frame memory 214, intra predictor 216, inter predictor218, prediction controller 220, and loop filter 212 included in decoder200 perform similar processes as performed by inverse quantizer 112,inverse transformer 114, adder 116, block memory 118, frame memory 122,intra predictor 124, inter predictor 126, prediction controller 128, andloop filter 120 included in encoder 100, respectively.

[Overall Flow of Decoding Process]

FIG. 69 is a flow chart illustrating one example of an overall decodingprocess performed by decoder 200.

First, splitting determiner 224 in decoder 200 determines a splittingpattern of each of a plurality of fixed-size blocks (128×128 pixels)included in a picture, based on a parameter which is input from entropydecoder 202 (Step Sp_1). This splitting pattern is a splitting patternselected by encoder 100. Decoder 200 then performs processes of StepsSp_2 to Sp_6 for each of a plurality of blocks of the splitting pattern.

Entropy decoder 202 decodes (specifically, entropy decodes) encodedquantized coefficients and a prediction parameter of a current block(Step Sp_2).

Next, inverse quantizer 204 performs inverse quantization of theplurality of quantized coefficients and inverse transformer 206 performsinverse transform of the result, to restore prediction residuals of thecurrent block (Step Sp_3).

Next, the prediction executor including all or part of intra predictor216, inter predictor 218, and prediction controller 220 generates aprediction image of the current block (Step Sp_4).

Next, adder 208 adds the prediction image to a prediction residual togenerate a reconstructed image (also referred to as a decoded imageblock) of the current block (Step Sp_5).

When the reconstructed image is generated, loop filter 212 performsfiltering of the reconstructed image (Step Sp_6).

Decoder 200 then determines whether decoding of the entire picture hasbeen finished (Step Sp_7). When determining that the decoding has notyet been finished (No in Step Sp_7), decoder 200 repeatedly executes theprocesses starting with Step Sp_1.

It is to be noted that the processes of these Steps Sp_1 to Sp_7 may beperformed sequentially by decoder 200, or two or more of the processesmay be performed in parallel. The processing order of the two or more ofthe processes may be modified.

[Splitting Determiner]

FIG. 70 is a diagram illustrating a relationship between splittingdeterminer 224 and other constituent elements. Splitting determiner 224may perform the following processes as examples.

For example, splitting determiner 224 collects block information fromblock memory 210 or frame memory 214, and furthermore obtains aparameter from entropy decoder 202. Splitting determiner 224 may thendetermine the splitting pattern of a fixed-size block, based on theblock information and the parameter. Splitting determiner 224 may thenoutput information indicating the determined splitting pattern toinverse transformer 206, intra predictor 216, and inter predictor 218.Inverse transformer 206 may perform inverse transform of transformcoefficients, based on the splitting pattern indicated by theinformation from splitting determiner 224. Intra predictor 216 and interpredictor 218 may generate a prediction image, based on the splittingpattern indicated by the information from splitting determiner 224.

[Entropy Decoder]

FIG. 71 is a block diagram illustrating one example of a configurationof entropy decoder 202.

Entropy decoder 202 generates quantized coefficients, a predictionparameter, and a parameter related to a splitting pattern, by entropydecoding the stream. For example, CABAC is used in the entropy decoding.More specifically, entropy decoder 202 includes, for example, binaryarithmetic decoder 202 a, context controller 202 b, and debinarizer 202c. Binary arithmetic decoder 202 a arithmetically decodes the streamusing a context value derived by context controller 202 b to a binarysignal. Context controller 202 b derives a context value according to afeature or a surrounding state of a syntax element, that is, anoccurrence probability of a binary signal, in the same manner asperformed by context controller 110 b of encoder 100. Debinarizer 202 cperforms debinarization for transforming the binary signal output frombinary arithmetic decoder 202 a to a multi-level signal indicatingquantized coefficients as described above. This binarization isperformed according to the binarization method described above.

With this, entropy decoder 202 outputs quantized coefficients of eachblock to inverse quantizer 204. Entropy decoder 202 may output aprediction parameter included in a stream (see FIG. 1 ) to intrapredictor 216, inter predictor 218, and prediction controller 220. Intrapredictor 216, inter predictor 218, and prediction controller 220 arecapable of executing the same prediction processes as those performed byintra predictor 124, inter predictor 126, and prediction controller 128at the encoder 100 side.

[Entropy Decoder]

FIG. 72 is a diagram illustrating a flow of CABAC in entropy decoder202.

First, initialization is performed in CABAC in entropy decoder 202. Inthe initialization, initialization in binary arithmetic decoder 202 aand setting of an initial context value are performed. Binary arithmeticdecoder 202 a and debinarizer 202 c then execute arithmetic decoding anddebinarization of, for example, encoded data of a CTU. At this time,context controller 202 b updates the context value each time arithmeticdecoding is performed. Context controller 202 b then saves the contextvalue as a post process. The saved context value is used, for example,to initialize the context value for the next CTU.

[Inverse Quantizer]

Inverse quantizer 204 inverse quantizes quantized coefficients of acurrent block which are inputs from entropy decoder 202. Morespecifically, inverse quantizer 204 inverse quantizes the quantizedcoefficients of the current block, based on quantization parameterscorresponding to the quantized coefficients. Inverse quantizer 204 thenoutputs the inverse quantized transform coefficients (that are transformcoefficients) of the current block to inverse transformer 206.

FIG. 73 is a block diagram illustrating one example of a configurationof inverse quantizer 204.

Inverse quantizer 204 includes, for example, quantization parametergenerator 204 a, predicted quantization parameter generator 204 b,quantization parameter storage 204 d, and inverse quantization executor204 e.

FIG. 74 is a flow chart illustrating one example of inverse quantizationperformed by inverse quantizer 204.

Inverse quantizer 204 may perform an inverse quantization process as oneexample for each CU based on the flow illustrated in FIG. 74 . Morespecifically, quantization parameter generator 204 a determines whetherto perform inverse quantization (Step Sv_11). Here, when determining toperform inverse quantization (Yes in Step Sv_11), quantization parametergenerator 204 a obtains a difference quantization parameter for thecurrent block from entropy decoder 202 (Step Sv_12).

Next, predicted quantization parameter generator 204 b then obtains aquantization parameter for a processing unit different from the currentblock from quantization parameter storage 204 d (Step Sv_13). Predictedquantization parameter generator 204 b generates a predictedquantization parameter of the current block based on the obtainedquantization parameter (Step Sv_14).

Quantization parameter generator 204 a then adds the differencequantization parameter for the current block obtained from entropydecoder 202 and the predicted quantization parameter for the currentblock generated by predicted quantization parameter generator 204 b(Step Sv_15). This addition generates a quantization parameter for thecurrent block. In addition, quantization parameter generator 204 astores the quantization parameter for the current block in quantizationparameter storage 204 d (Step Sv_16).

Next, inverse quantization executor 204 e inverse quantizes thequantized coefficients of the current block into transform coefficients,using the quantization parameter generated in Step Sv_15 (Step Sv_17).

It is to be noted that the difference quantization parameter may bedecoded at the bit sequence level, picture level, slice level, bricklevel, or CTU level. In addition, the initial value of the quantizationparameter may be decoded at the sequence level, picture level, slicelevel, brick level, or CTU level. At this time, the quantizationparameter may be generated using the initial value of the quantizationparameter and the difference quantization parameter.

It is to be noted that inverse quantizer 204 may include a plurality ofinverse quantizers, and may inverse quantize the quantized coefficientsusing an inverse quantization method selected from a plurality ofinverse quantization methods.

[Inverse Transformer]

Inverse transformer 206 restores prediction residuals by inversetransforming the transform coefficients which are inputs from inversequantizer 204.

For example, when information parsed from a stream indicates that EMT orAMT is to be applied (for example, when an AMT flag is true), inversetransformer 206 inverse transforms the transform coefficients of thecurrent block based on information indicating the parsed transform type.

Moreover, for example, when information parsed from a stream indicatesthat NSST is to be applied, inverse transformer 206 applies a secondaryinverse transform to the transform coefficients.

FIG. 75 is a flow chart illustrating one example of a process performedby inverse transformer 206.

For example, inverse transformer 206 determines whether informationindicating that no orthogonal transform is performed is present in astream (Step St_11). Here, when determining that no such information ispresent (No in Step St_11), inverse transformer 206 obtains informationindicating the transform type decoded by entropy decoder 202 (StepSt_12). Next, based on the information, inverse transformer 206determines the transform type used for the orthogonal transform inencoder 100 (Step St_13). Inverse transformer 206 then performs inverseorthogonal transform using the determined transform type (Step St_14).

FIG. 76 is a flow chart illustrating another example of a processperformed by inverse transformer 206.

For example, inverse transformer 206 determines whether a transform sizeis smaller than or equal to a predetermined value (Step Su_11). Here,when determining that the transform size is smaller than or equal to apredetermined value (Yes in Step Su_11), inverse transformer 206obtains, from entropy decoder 202, information indicating whichtransform type has been used by encoder 100 among at least one transformtype included in the first transform type group (Step Su_12). It is tobe noted that such information is decoded by entropy decoder 202 andoutput to inverse transformer 206.

Based on the information, inverse transformer 206 determines thetransform type used for the orthogonal transform in encoder 100 (StepSu_13). Inverse transformer 206 then inverse orthogonal transforms thetransform coefficients of the current block using the determinedtransform type (Step Su_14). When determining that a transform size isnot smaller than or equal to the predetermined value (No in Step Su_11),inverse transformer 206 inverse transforms the transform coefficients ofthe current block using the second transform type group (Step Su_15).

It is to be noted that the inverse orthogonal transform by inversetransformer 206 may be performed according to the flow illustrated inFIG. 75 or FIG. 76 for each TU as one example. In addition, inverseorthogonal transform may be performed by using a predefined transformtype without decoding information indicating a transform type used fororthogonal transform. In addition, the transform type is specificallyDST7, DCT8, or the like. In inverse orthogonal transform, an inversetransform basis function corresponding to the transform type is used.

[Adder]

Adder 208 reconstructs the current block by adding a prediction residualwhich is an input from inverse transformer 206 and a prediction imagewhich is an input from prediction controller 220. In other words, areconstructed image of the current block is generated. Adder 208 thenoutputs the reconstructed image of the current block to block memory 210and loop filter 212.

[Block Memory]

Block memory 210 is storage for storing a block which is included in acurrent picture and is referred to in intra prediction. Morespecifically, block memory 210 stores a reconstructed image output fromadder 208.

[Loop Filter]

Loop filter 212 applies a loop filter to the reconstructed imagegenerated by adder 208, and outputs the filtered reconstructed image toframe memory 214 and a display device, etc.

When information indicating ON or OFF of an ALF parsed from a streamindicates that an ALF is ON, one filter from among a plurality offilters is selected based on the direction and activity of localgradients, and the selected filter is applied to the reconstructedimage.

FIG. 77 is a block diagram illustrating one example of a configurationof loop filter 212. It is to be noted that loop filter 212 has aconfiguration similar to the configuration of loop filter 120 of encoder100.

For example, as illustrated in FIG. 77 , loop filter 212 includesdeblocking filter executor 212 a, SAO executor 212 b, and ALF executor212 c. Deblocking filter executor 212 a performs a deblocking filterprocess of the reconstructed image. SAO executor 212 b performs a SAOprocess of the reconstructed image after being subjected to thedeblocking filter process. ALF executor 212 c performs an ALF process ofthe reconstructed image after being subjected to the SAO process. It isto be noted that loop filter 212 does not always need to include all theconstituent elements disclosed in FIG. 77 , and may include only part ofthe constituent elements. In addition, loop filter 212 may be configuredto perform the above processes in a processing order different from theone disclosed in FIG. 77 .

[Frame Memory]

Frame memory 214 is, for example, storage for storing reference picturesfor use in inter prediction, and is also referred to as a frame buffer.More specifically, frame memory 214 stores a reconstructed imagefiltered by loop filter 212.

[Predictor (Intra Predictor, Inter Predictor, Prediction Controller)]

FIG. 78 is a flow chart illustrating one example of a process performedby a predictor of decoder 200. It is to be noted that the predictionexecutor includes all or part of the following constituent elements:intra predictor 216; inter predictor 218; and prediction controller 220.The prediction executor includes, for example, intra predictor 216 andinter predictor 218.

The predictor generates a prediction image of a current block (StepSq_1). This prediction image is also referred to as a prediction signalor a prediction block. It is to be noted that the prediction signal is,for example, an intra prediction signal or an inter prediction signal.More specifically, the predictor generates the prediction image of thecurrent block using a reconstructed image which has been alreadyobtained for another block through generation of a prediction image,restoration of a prediction residual, and addition of a predictionimage. The predictor of decoder 200 generates the same prediction imageas the prediction image generated by the predictor of encoder 100. Inother words, the prediction images are generated according to a methodcommon between the predictors or mutually corresponding methods.

The reconstructed image may be, for example, an image in a referencepicture, or an image of a decoded block (that is, the other blockdescribed above) in a current picture which is the picture including thecurrent block. The decoded block in the current picture is, for example,a neighboring block of the current block.

FIG. 79 is a flow chart illustrating another example of a processperformed by the predictor of decoder 200.

The predictor determines either a method or a mode for generating aprediction image (Step Sr_1). For example, the method or mode may bedetermined based on, for example, a prediction parameter, etc.

When determining a first method as a mode for generating a predictionimage, the predictor generates a prediction image according to the firstmethod (Step Sr_2 a). When determining a second method as a mode forgenerating a prediction image, the predictor generates a predictionimage according to the second method (Step Sr_2 b). When determining athird method as a mode for generating a prediction image, the predictorgenerates a prediction image according to the third method (Step Sr_2c).

The first method, the second method, and the third method may bemutually different methods for generating a prediction image. Each ofthe first to third methods may be an inter prediction method, an intraprediction method, or another prediction method. The above-describedreconstructed image may be used in these prediction methods.

FIG. 80A and FIG. 80B illustrate a flow chart illustrating anotherexample of a process performed by a predictor of decoder 200.

The predictor may perform a prediction process according to the flowillustrated in FIG. 80A and FIG. 80B as one example. It is to be notedthat intra block copy illustrated in FIG. 80A and FIG. 80B is one modewhich belongs to inter prediction, and in which a block included in acurrent picture is referred to as a reference image or a referenceblock. In other words, no picture different from the current picture isreferred to in intra block copy. In addition, the PCM mode illustratedin FIG. 80A is one mode which belongs to intra prediction, and in whichno transform and quantization is performed.

[Intra Predictor]

Intra predictor 216 performs intra prediction by referring to a block ina current picture stored in block memory 210, based on the intraprediction mode parsed from the stream, to generate a prediction imageof a current block (that is, an intra prediction image). Morespecifically, intra predictor 216 performs intra prediction by referringto pixel values (for example, luma and/or chroma values) of a block orblocks neighboring the current block to generate an intra predictionimage, and then outputs the intra prediction image to predictioncontroller 220.

It is to be noted that when an intra prediction mode in which a lumablock is referred to in intra prediction of a chroma block is selected,intra predictor 216 may predict the chroma component of the currentblock based on the luma component of the current block.

Moreover, when information parsed from a stream indicates that PDPC isto be applied, intra predictor 216 corrects intra predicted pixel valuesbased on horizontal/vertical reference pixel gradients.

FIG. 81 is a diagram illustrating one example of a process performed byintra predictor 216 of decoder 200.

Intra predictor 216 firstly determines whether an MPM flag indicating 1is present in the stream (Step Sw_11). Here, when determining that theMPM flag indicating 1 is present (Yes in Step Sw_11), intra predictor216 obtains, from entropy decoder 202, information indicating the intraprediction mode selected in encoder 100 among MPMs (Step Sw_12). It isto be noted that such information is decoded by entropy decoder 202 andoutput to intra predictor 216. Next, intra predictor 216 determines anMPM (Step Sw_13). MPMs include, for example, six intra prediction modes.Intra predictor 216 then determines the intra prediction mode which isincluded in a plurality of intra prediction modes included in the MPMsand is indicated by the information obtained in Step Sw_12 (Step Sw_14).

When determining that no MPM flag indicating 1 is present (No in StepSw_11), intra predictor 216 obtains information indicating the intraprediction mode selected in encoder 100 (Step Sw_15). In other words,intra predictor 216 obtains, from entropy decoder 202, informationindicating the intra prediction mode selected in encoder 100 from amongat least one intra prediction mode which is not included in the MPMs. Itis to be noted that such information is decoded by entropy decoder 202and output to intra predictor 216. Intra predictor 216 then determinesthe intra prediction mode which is not included in a plurality of intraprediction modes included in the MPMs and is indicated by theinformation obtained in Step Sw_15 (Step Sw_17).

Intra predictor 216 generates a prediction image according to the intraprediction mode determined in Step Sw_14 or Step Sw_17 (Step Sw_18).

[Inter Predictor]

Inter predictor 218 predicts the current block by referring to areference picture stored in frame memory 214. Prediction is performed inunits of a current block or a current sub-block in the current block. Itis to be noted that the sub-block is included in the block and is a unitsmaller than the block. The size of the sub-block may be 4×4 pixels, 8×8pixels, or another size. The size of the sub-block may be switched for aunit such as a slice, brick, picture, etc.

For example, inter predictor 218 generates an inter prediction image ofa current block or a current sub-block by performing motion compensationusing motion information (for example, an MV) parsed from a stream (forexample, a prediction parameter output from entropy decoder 202), andoutputs the inter prediction image to prediction controller 220.

When the information parsed from the stream indicates that the OBMC modeis to be applied, inter predictor 218 generates the inter predictionimage using motion information of a neighboring block in addition tomotion information of the current block obtained through motionestimation.

Moreover, when the information parsed from the stream indicates that theFRUC mode is to be applied, inter predictor 218 derives motioninformation by performing motion estimation in accordance with thepattern matching method (bilateral matching or template matching) parsedfrom the stream. Inter predictor 218 then performs motion compensation(prediction) using the derived motion information.

Moreover, when the BIO mode is to be applied, inter predictor 218derives an MV based on a model assuming uniform linear motion. Inaddition, when the information parsed from the stream indicates that theaffine mode is to be applied, inter predictor 218 derives an MV for eachsub-block, based on the MVs of a plurality of neighboring blocks.

[MV Derivation Flow]

FIG. 82 is a flow chart illustrating one example of MV derivation indecoder 200.

Inter predictor 218 determines, for example, whether to decode motioninformation (for example, an MV). For example, inter predictor 218 maymake the determination according to the prediction mode included in thestream, or may make the determination based on other informationincluded in the stream. Here, when determining to decode motioninformation, inter predictor 218 derives an MV for a current block in amode in which the motion information is decoded. When determining not todecode motion information, inter predictor 218 derives an MV in a modein which no motion information is decoded.

Here, MV derivation modes include a normal inter mode, a normal mergemode, a FRUC mode, an affine mode, etc. which are described later. Modesin which motion information is decoded among the modes include thenormal inter mode, the normal merge mode, the affine mode (specifically,an affine inter mode and an affine merge mode), etc. It is to be notedthat motion information may include not only an MV but also MV predictorselection information which is described later. Modes in which no motioninformation is decoded include the FRUC mode, etc. Inter predictor 218selects a mode for deriving an MV for the current block from theplurality of modes, and derives the MV for the current block using theselected mode.

FIG. 83 is a flow chart illustrating another example of MV derivation indecoder 200.

For example, inter predictor 218 may determine whether to decode an MVdifference, that is for example, may make the determination according tothe prediction mode included in the stream, or may make thedetermination based on other information included in the stream. Here,when determining to decode an MV difference, inter predictor 218 mayderive an MV for a current block in a mode in which the MV difference isdecoded. In this case, for example, the MV difference included in thestream is decoded as a prediction parameter.

When determining not to decode any MV difference, inter predictor 218derives an MV in a mode in which no MV difference is decoded. In thiscase, no encoded MV difference is included in the stream.

Here, as described above, the MV derivation modes include the normalinter mode, the normal merge mode, the FRUC mode, the affine mode, etc.which are described later. Modes in which an MV difference is encodedamong the modes include the normal inter mode and the affine mode(specifically, the affine inter mode), etc. Modes in which no MVdifference is encoded include the FRUC mode, the normal merge mode, theaffine mode (specifically, the affine merge mode), etc. Inter predictor218 selects a mode for deriving an MV for the current block from theplurality of modes, and derives the MV for the current block using theselected mode.

[MV Derivation>Normal Inter Mode]

For example, when information parsed from a stream indicates that thenormal inter mode is to be applied, inter predictor 218 derives an MVbased on the information parsed from the stream and performs motioncompensation (prediction) using the MV.

FIG. 84 is a flow chart illustrating an example of inter prediction bynormal inter mode in decoder 200.

Inter predictor 218 of decoder 200 performs motion compensation for eachblock. At this time, first, inter predictor 218 obtains a plurality ofMV candidates for a current block based on information such as MVs of aplurality of decoded blocks temporally or spatially surrounding thecurrent block (Step Sg_11). In other words, inter predictor 218generates an MV candidate list.

Next, inter predictor 218 extracts N (an integer of 2 or larger) MVcandidates from the plurality of MV candidates obtained in Step Sg_11,as motion vector predictor candidates (also referred to as MV predictorcandidates) according to the predetermined ranks in priority order (StepSg_12). It is to be noted that the ranks in priority order aredetermined in advance for the respective N MV predictor candidates.

Next, inter predictor 218 decodes the MV predictor selection informationfrom the input stream, and selects one MV predictor candidate from the NMV predictor candidates as the MV predictor for the current block usingthe decoded MV predictor selection information (Step Sg_13).

Next, inter predictor 218 decodes an MV difference from the inputstream, and derives an MV for the current block by adding a differencevalue which is the decoded MV difference and the selected MV predictor(Step Sg_14).

Lastly, inter predictor 218 generates a prediction image for the currentblock by performing motion compensation of the current block using thederived MV and the decoded reference picture (Step Sg_15). The processesin Steps Sg_11 to Sg_15 are executed on each block. For example, whenthe processes in Steps Sg_11 to Sg_15 are executed on each of all theblocks in the slice, inter prediction of the slice using the normalinter mode finishes. For example, when the processes in Steps Sg_11 toSg_15 are executed on each of all the blocks in the picture, interprediction of the picture using the normal inter mode finishes. It is tobe noted that not all the blocks included in the slice may be subjectedto the processes in Steps Sg_11 to Sg_15, and inter prediction of theslice using the normal inter mode may finish when part of the blocks aresubjected to the processes. Likewise, inter prediction of the pictureusing the normal inter mode may finish when the processes in Steps Sg_11to Sg_15 are executed on part of the blocks in the picture.

[MV Derivation>Normal Merge Mode]

For example, when information parsed from a stream indicates that thenormal merge mode is to be applied, inter predictor 218 derives an MVand performs motion compensation (prediction) using the MV.

FIG. 85 is a flow chart illustrating an example of inter prediction bynormal merge mode in decoder 200.

At this time, first, inter predictor 218 obtains a plurality of MVcandidates for a current block based on information such as MVs of aplurality of decoded blocks temporally or spatially surrounding thecurrent block (Step Sh_11). In other words, inter predictor 218generates an MV candidate list.

Next, inter predictor 218 selects one MV candidate from the plurality ofMV candidates obtained in Step Sh_11, thereby deriving an MV for thecurrent block (Step Sh_12). More specifically, inter predictor 218obtains MV selection information included as a prediction parameter in astream, and selects the MV candidate identified by the MV selectioninformation as the MV for the current block.

Lastly, inter predictor 218 generates a prediction image for the currentblock by performing motion compensation of the current block using thederived MV and the decoded reference picture (Step Sh_13). The processesin Steps Sh_11 to Sh_13 are executed, for example, on each block. Forexample, when the processes in Steps Sh_11 to Sh_13 are executed on eachof all the blocks in the slice, inter prediction of the slice using thenormal merge mode finishes. In addition, when the processes in StepsSh_11 to Sh_13 are executed on each of all the blocks in the picture,inter prediction of the picture using the normal merge mode finishes. Itis to be noted that not all the blocks included in the slice aresubjected to the processes in Steps Sh_11 to Sh_13, and inter predictionof the slice using the normal merge mode may finish when part of theblocks are subjected to the processes. Likewise, inter prediction of thepicture using the normal merge mode may finish when the processes inSteps Sh_11 to Sh_13 are executed on part of the blocks in the picture.

[MV Derivation>FRUC Mode]

For example, when information parsed from a stream indicates that theFRUC mode is to be applied, inter predictor 218 derives an MV in theFRUC mode and performs motion compensation (prediction) using the MV. Inthis case, the motion information is derived at the decoder 200 sidewithout being signaled from the encoder 100 side. For example, decoder200 may derive the motion information by performing motion estimation.In this case, decoder 200 performs motion estimation without using anypixel value in a current block.

FIG. 86 is a flow chart illustrating an example of inter prediction byFRUC mode in decoder 200.

First, inter predictor 218 generates a list indicating MVs of decodedblocks spatially or temporally neighboring the current block byreferring to the MVs as MV candidates (the list is an MV candidate list,and may be used also as an MV candidate list for normal merge mode (StepSi_11). Next, a best MV candidate is selected from the plurality of MVcandidates registered in the MV candidate list (Step Si_12). Forexample, inter predictor 218 calculates the evaluation value of each MVcandidate included in the MV candidate list, and selects one of the MVcandidates as the best MV candidate based on the evaluation values.Based on the selected best MV candidate, inter predictor 218 thenderives an MV for the current block (Step Si_14). More specifically, forexample, the selected best MV candidate is directly derived as the MVfor the current block. In addition, for example, the MV for the currentblock may be derived using pattern matching in a surrounding region of aposition which is included in a reference picture and corresponds to theselected best MV candidate. In other words, estimation using the patternmatching in a reference picture and the evaluation values may beperformed in the surrounding region of the best MV candidate, and whenthere is an MV that yields a better evaluation value, the best MVcandidate may be updated to the MV that yields the better evaluationvalue, and the updated MV may be determined as the final MV for thecurrent block. Update to the MV that yields the better evaluation valuemay not be performed.

Lastly, inter predictor 218 generates a prediction image for the currentblock by performing motion compensation of the current block using thederived MV and the decoded reference picture (Step Si_15). The processesin Steps Si_11 to Si_15 are executed, for example, on each block. Forexample, when the processes in Steps Si_11 to Si_15 are executed on eachof all the blocks in the slice, inter prediction of the slice using theFRUC mode finishes. For example, when the processes in Steps Si_11 toSi_15 are executed on each of all the blocks in the picture, interprediction of the picture using the FRUC mode finishes. Each sub-blockmay be processed similarly to the above-described case of processingeach block.

[MV Derivation>Affine Merge Mode]

For example, when information parsed from a stream indicates that theaffine merge mode is to be applied, inter predictor 218 derives an MV inthe affine merge mode and performs motion compensation (prediction)using the MV.

FIG. 87 is a flow chart illustrating an example of inter prediction bythe affine merge mode in decoder 200.

In the affine merge mode, first, inter predictor 218 derives MVs atrespective control points for a current block (Step Sk_11). The controlpoints are an upper-left corner point of the current block and anupper-right corner point of the current block as illustrated in FIG.46A, or an upper-left corner point of the current block, an upper-rightcorner point of the current block, and a lower-left corner point of thecurrent block as illustrated in FIG. 46B.

For example, when the MV derivation methods illustrated in FIGS. 47A to47C are used, as illustrated in FIG. 47A, inter predictor 218 checksdecoded block A (left), block B (upper), block C (upper-right), block D(lower-left), and block E (upper-left) in this order, and identifies thefirst effective block decoded according to the affine mode.

Inter predictor 218 derives the MV at the control point using theidentified first effective block decoded according to the affine mode.For example, when block A is identified and block A has two controlpoints, as illustrated in FIG. 47B, inter predictor 218 calculatesmotion vector v₀ at the upper-left corner control point of the currentblock and motion vector v₁ at the upper-right corner control point ofthe current block by projecting motion vectors v₃ and v₄ at theupper-left corner and the upper-right corner of the decoded blockincluding block A onto the current block. In this way, the MV at eachcontrol point is derived.

It is to be noted that, as illustrated in FIG. 49A, MVs at three controlpoints may be calculated when block A is identified and block A has twocontrol points, and that, as illustrated in FIG. 49B, MVs at two controlpoints may be calculated when block A is identified and when block A hasthree control points.

In addition, when MV selection information is included as a predictionparameter in a stream, inter predictor 218 may derive the MV at eachcontrol point for the current block using the MV selection information.

Next, inter predictor 218 performs motion compensation of each of aplurality of sub-blocks included in the current block. In other words,inter predictor 218 calculates an MV for each of the plurality ofsub-blocks as an affine MV, using either two motion vectors v₀ and v₁and the above expression (1A) or three motion vectors v₀, v₁, and v₂ andthe above expression (1B) (Step Sk_12). Inter predictor 218 thenperforms motion compensation of the sub-blocks using these affine MVsand decoded reference pictures (Step Sk_13). When the processes in StepsSk_12 and Sk_13 are executed for each of all the sub-blocks included inthe current block, the inter prediction using the affine merge mode forthe current block finishes. In other words, motion compensation of thecurrent block is performed to generate a prediction image of the currentblock.

It is to be noted that the above-described MV candidate list may begenerated in Step Sk_11. The MV candidate list may be, for example, alist including MV candidates derived using a plurality of MV derivationmethods for each control point. The plurality of MV derivation methodsmay be any combination of the MV derivation methods illustrated in FIGS.47A to 47C, the MV derivation methods illustrated in FIGS. 48A and 48B,the MV derivation methods illustrated in FIGS. 49A and 49B, and other MVderivation methods.

It is to be noted that an MV candidate list may include MV candidates ina mode in which prediction is performed in units of a sub-block, otherthan the affine mode.

It is to be noted that, for example, an MV candidate list including MVcandidates in an affine merge mode in which two control points are usedand an affine merge mode in which three control points are used may begenerated as an MV candidate list. Alternatively, an MV candidate listincluding MV candidates in the affine merge mode in which two controlpoints are used and an MV candidate list including MV candidates in theaffine merge mode in which three control points are used may begenerated separately. Alternatively, an MV candidate list including MVcandidates in one of the affine merge mode in which two control pointsare used and the affine merge mode in which three control points areused may be generated.

[MV Derivation>Affine Inter Mode]

For example, when information parsed from a stream indicates that theaffine inter mode is to be applied, inter predictor 218 derives an MV inthe affine inter mode and performs motion compensation (prediction)using the MV.

FIG. 88 is a flow chart illustrating an example of inter prediction bythe affine inter mode in decoder 200.

In the affine inter mode, first, inter predictor 218 derives MVpredictors (v₀, v₁) or (v₀, v₁, v₂) of respective two or three controlpoints for a current block (Step Sj_11). The control points are anupper-left corner point of the current block, an upper-right cornerpoint of the current block, and a lower-left corner point of the currentblock as illustrated in FIG. 46A or FIG. 46B.

Inter predictor 218 obtains MV predictor selection information includedas a prediction parameter in the stream, and derives the MV predictor ateach control point for the current block using the MV identified by theMV predictor selection information. For example, when the MV derivationmethods illustrated in FIGS. 48A and 48B are used, inter predictor 218derives the motion vector predictors (v₀, v₁) or (v₀, v₁, v₂) at controlpoints for the current block by selecting the MV of the block identifiedby the MV predictor selection information among decoded blocks in thevicinity of the respective control points for the current blockillustrated in either FIG. 48A or FIG. 48B.

Next, inter predictor 218 obtains each MV difference included as aprediction parameter in the stream, and adds the MV predictor at eachcontrol point for the current block and the MV difference correspondingto the MV predictor (Step Sj_12). In this way, the MV at each controlpoint for the current block is derived.

Next, inter predictor 218 performs motion compensation of each of aplurality of sub-blocks included in the current block. In other words,inter predictor 218 calculates an MV for each of the plurality ofsub-blocks as an affine MV, using either two motion vectors v₀ and v₁and the above expression (1A) or three motion vectors v₀, v₁, and v₂ andthe above expression (1B) (Step Sj_13). Inter predictor 218 thenperforms motion compensation of the sub-blocks using these affine MVsand decoded reference pictures (Step Sj_14). When the processes in StepsSj_13 and Sj_14 are executed for each of all the sub-blocks included inthe current block, the inter prediction using the affine merge mode forthe current block finishes. In other words, motion compensation of thecurrent block is performed to generate a prediction image of the currentblock.

It is to be noted that the above-described MV candidate list may begenerated in Step Sj_11 as in Step Sk_11.

[MV Derivation>Triangle Mode]

For example, when information parsed from a stream indicates that thetriangle mode is to be applied, inter predictor 218 derives an MV in thetriangle mode and performs motion compensation (prediction) using theMV.

FIG. 89 is a flow chart illustrating an example of inter prediction bythe triangle mode in decoder 200.

In the triangle mode, first, inter predictor 218 splits the currentblock into a first partition and a second partition (Step Sx_11). Atthis time, inter predictor 218 may obtain, from the stream, partitioninformation which is information related to the splitting as aprediction parameter. Inter predictor 218 may then split a current blockinto a first partition and a second partition according to the partitioninformation.

Next, first, inter predictor 218 obtains a plurality of MV candidatesfor a current block based on information such as MVs of a plurality ofdecoded blocks temporally or spatially surrounding the current block(Step Sx_12). In other words, inter predictor 218 generates an MVcandidate list.

Inter predictor 218 then selects the MV candidate for the firstpartition and the MV candidate for the second partition as a first MVand a second MV, respectively, from the plurality of MV candidatesobtained in Step Sx_11 (Step Sx_13). At this time, inter predictor 218may obtain, from the stream, MV selection information for identifyingeach selected MV candidate, as a prediction parameter. Inter predictor218 may then select the first MV and the second MV according to the MVselection information.

Next, inter predictor 218 generates a first prediction image byperforming motion compensation using the selected first MV and a decodedreference picture (Step Sx_14). Likewise, inter predictor 218 generatesa second prediction image by performing motion compensation using theselected second MV and a decoded reference picture (Step Sx_15).

Lastly, inter predictor 218 generates a prediction image for the currentblock by performing a weighted addition of the first prediction imageand the second prediction image (Step Sx_16).

[Motion Estimation>DMVR]

For example, information parsed from a stream indicates that DMVR is tobe applied, inter predictor 218 performs motion estimation using DMVR.

FIG. 90 is a flow chart illustrating an example of motion estimation byDMVR in decoder 200.

Inter predictor 218 derives an MV for a current block according to themerge mode (Step Sl_11). Next, inter predictor 218 derives the final MVfor the current block by searching the region surrounding the referencepicture indicated by the MV derived in Sl_11 (Step Sl_12). In otherwords, the MV of the current block is determined according to the DMVR.

FIG. 91 is a flow chart illustrating a specific example of motionestimation by DMVR in decoder 200.

First, in Step 1 illustrated in FIG. 58A, inter predictor 218 calculatesthe cost between the search position (also referred to as a startingpoint) indicated by the initial MV and eight surrounding searchpositions. Inter predictor 218 then determines whether the cost at eachof the search positions other than the starting point is the smallest.Here, when determining that the cost at one of the search positionsother than the starting point is the smallest, inter predictor 218changes a target to the search position at which the smallest cost isobtained, and performs the process in Step 2 illustrated in FIG. 58A.When the cost at the starting point is the smallest, inter predictor 218skips the process in Step 2 illustrated in FIG. 58A and performs theprocess in Step 3.

In Step 2 illustrated in FIG. 58A, inter predictor 218 performs searchsimilar to the process in Step 1, regarding the search position afterthe target change as a new starting point according to the result of theprocess in Step 1. Inter predictor 218 then determines whether the costat each of the search positions other than the starting point is thesmallest. Here, when determining that the cost at one of the searchpositions other than the starting point is the smallest, inter predictor218 performs the process in Step 4. When the cost at the starting pointis the smallest, inter predictor 218 performs the process in Step 3.

In Step 4, inter predictor 218 regards the search position at thestarting point as the final search position, and determines thedifference between the position indicated by the initial MV and thefinal search position to be a vector difference.

In Step 3 illustrated in FIG. 58A, inter predictor 218 determines thepixel position at sub-pixel accuracy at which the smallest cost isobtained, based on the costs at the four points located at upper, lower,left, and right positions with respect to the starting point in Step 1or Step 2, and regards the pixel position as the final search position.The pixel position at the sub-pixel accuracy is determined by performingweighted addition of each of the four upper, lower, left, and rightvectors ((0, 1), (0, −1), (−1, 0), and (1, 0)), using, as a weight, thecost at a corresponding one of the four search positions. Interpredictor 218 then determines the difference between the positionindicated by the initial MV and the final search position to be thevector difference.

[Motion Compensation>BIO/OBMC/LIC]

For example, when information parsed from a stream indicates thatcorrection of a prediction image is to be performed, upon generating aprediction image, inter predictor 218 corrects the prediction imagebased on the mode for the correction. The mode is, for example, one ofBIO, OBMC, and LIC described above.

FIG. 92 is a flow chart illustrating one example of generation of aprediction image in decoder 200.

Inter predictor 218 generates a prediction image (Step Sm_11), andcorrects the prediction image according to any of the modes describedabove (Step Sm_12).

FIG. 93 is a flow chart illustrating another example of generation of aprediction image in decoder 200.

Inter predictor 218 derives an MV for a current block (Step Sn_11).Next, inter predictor 218 generates a prediction image using the MV(Step Sn_12), and determines whether to perform a correction process(Step Sn_13). For example, inter predictor 218 obtains a predictionparameter included in the stream, and determines whether to perform acorrection process based on the prediction parameter. This predictionparameter is, for example, a flag indicating whether each of theabove-described modes is to be applied. Here, when determining toperform a correction process (Yes in Step Sn_13), inter predictor 218generates the final prediction image by correcting the prediction image(Step Sn_14). It is to be noted that, in LIC, the luminance andchrominance of the prediction image may be corrected in Step Sn_14. Whendetermining not to perform a correction process (No in Step Sn_13),inter predictor 218 outputs the final prediction image withoutcorrecting the prediction image (Step Sn_15).

[Motion Compensation>OBMC]

For example, when information parsed from a stream indicates that OBMCis to be performed, upon generating a prediction image, inter predictor218 corrects the prediction image according to the OBMC.

FIG. 94 is a flow chart illustrating an example of correction of aprediction image by OBMC in decoder 200. It is to be noted that the flowchart in FIG. 94 indicates the correction flow of a prediction imageusing the current picture and the reference picture illustrated in FIG.62 .

First, as illustrated in FIG. 62 , inter predictor 218 obtains aprediction image (Pred) by normal motion compensation using an MVassigned to the current block.

Next, inter predictor 218 obtains a prediction image (Pred_L) byapplying a motion vector (MV_L) which has been already derived for thedecoded block neighboring to the left of the current block to thecurrent block (re-using the motion vector for the current block). Interpredictor 218 then performs a first correction of a prediction image byoverlapping two prediction images Pred and Pred_L. This provides aneffect of blending the boundary between neighboring blocks.

Likewise, inter predictor 218 obtains a prediction image (Pred_U) byapplying an MV (MV_U) which has been already derived for the decodedblock neighboring above the current block to the current block (re-usingthe motion vector for the current block). Inter predictor 218 thenperforms a second correction of the prediction image by overlapping theprediction image Pred_U to the prediction images (for example, Pred andPred_L) on which the first correction has been performed. This providesan effect of blending the boundary between neighboring blocks. Theprediction image obtained by the second correction is the one in whichthe boundary between the neighboring blocks has been blended (smoothed),and thus is the final prediction image of the current block.

[Motion Compensation>BIO]

For example, when information parsed from a stream indicates that BIO isto be performed, upon generating a prediction image, inter predictor 218corrects the prediction image according to the BIO.

FIG. 95 is a flow chart illustrating an example of correction of aprediction image by the BIO in decoder 200.

As illustrated in FIG. 63 , inter predictor 218 derives two motionvectors (M0, M1), using two reference pictures (Ref₀, Ref₁) differentfrom the picture (Cur Pic) including a current block. Inter predictor218 then derives a prediction image for the current block using the twomotion vectors (M0, M1) (Step Sy_11). It is to be noted that motionvector M0 is a motion vector (MVx₀, MVy₀) corresponding to referencepicture Ref₀, and motion vector M1 is a motion vector (MVx₁, MVy₁)corresponding to reference picture Ref₁.

Next, inter predictor 218 derives interpolated image I⁰ for the currentblock using motion vector M0 and reference picture L0. In addition,inter predictor 218 derives interpolated image I¹ for the current blockusing motion vector M1 and reference picture L1 (Step Sy_12). Here,interpolated image I⁰ is an image included in reference picture Ref₀ andto be derived for the current block, and interpolated image I¹ is animage included in reference picture Ref₁ and to be derived for thecurrent block. Each of interpolated image I⁰ and interpolated image I¹may be the same in size as the current block. Alternatively, each ofinterpolated image I⁰ and interpolated image I¹ may be an image largerthan the current block. Furthermore, interpolated image I⁰ andinterpolated image I¹ may include a prediction image obtained by usingmotion vectors (M0, M1) and reference pictures (L0, L1) and applying amotion compensation filter.

In addition, inter predictor 218 derives gradient images (Ix⁰, Ix¹, Iy⁰,Iy¹) of the current block, from interpolated image I⁰ and interpolatedimage II (Step Sy_13). It is to be noted that the gradient images in thehorizontal direction are (Ix⁰, Ix¹), and the gradient images in thevertical direction are (Iy⁰, Iy¹). Inter predictor 218 may derive thegradient images by, for example, applying a gradient filter to theinterpolated images. The gradient images may be the ones each of whichindicates the amount of spatial change in pixel value along thehorizontal direction or the amount of spatial change in pixel valuealong the vertical direction.

Next, inter predictor 218 derives, for each sub-block of the currentblock, an optical flow (v_(x), v_(y)) which is a velocity vector, usingthe interpolated images (I⁰, I¹) and the gradient images (Ix⁰, Ix¹, Iy⁰,Iy¹). As one example, a sub-block may be 4×4 pixel sub-CU.

Next, inter predictor 218 corrects a prediction image for the currentblock using the optical flow (v_(x), v_(y)). For example, interpredictor 218 derives a correction value for the value of a pixelincluded in a current block, using the optical flow (v_(x), v_(y)) (StepSy_15). Inter predictor 218 may then correct the prediction image forthe current block using the correction value (Step Sy_16). It is to benoted that the correction value may be derived in units of a pixel, ormay be derived in units of a plurality of pixels or in units of asub-block.

It is to be noted that the BIO process flow is not limited to theprocess disclosed in FIG. 95 . Only part of the processes disclosed inFIG. 95 may be performed, or a different process may be added or used asa replacement, or the processes may be executed in a differentprocessing order.

[Motion Compensation>LIC]

For example, when information parsed from a stream indicates that LIC isto be performed, upon generating a prediction image, inter predictor 218corrects the prediction image according to the LIC.

FIG. 96 is a flow chart illustrating an example of correction of aprediction image by the LIC in decoder 200.

First, inter predictor 218 obtains a reference image corresponding to acurrent block from a decoded reference picture using an MV (Step Sz_11).

Next, inter predictor 218 extracts, for the current block, informationindicating how the luma value has changed between the current pictureand the reference picture (Step Sz_12). This extraction is performedbased on the luma pixel values for the decoded left neighboringreference region (surrounding reference region) and the decoded upperneighboring reference region (surrounding reference region), and theluma pixel values at the corresponding positions in the referencepicture specified by the derived MVs. Inter predictor 218 calculates aluminance correction parameter, using the information indicating how theluma value changed (Step Sz_13).

Inter predictor 218 generates a prediction image for the current blockby performing a luminance correction process in which the luminancecorrection parameter is applied to the reference image in the referencepicture specified by the MV (Step Sz_14). In other words, the predictionimage which is the reference image in the reference picture specified bythe MV is subjected to the correction based on the luminance correctionparameter. In this correction, luminance may be corrected, orchrominance may be corrected.

[Prediction Controller]

Prediction controller 220 selects either an intra prediction image or aninter prediction image, and outputs the selected image to adder 208. Asa whole, the configurations, functions, and processes of predictioncontroller 220, intra predictor 216, and inter predictor 218 at thedecoder 200 side may correspond to the configurations, functions, andprocesses of prediction controller 128, intra predictor 124, and interpredictor 126 at the encoder 100 side.

[Description of Intra Block Copy (IBC) Mode]

Inter predictor 126 (see FIG. 7 ) of encoder 100 according to a firstaspect can refer to a decoded pixel (that is, a pixel that has beenencoded and decoded) in a picture which includes a current block (thatis, a current picture), in addition to past and future pictures. Forexample, inter predictor 126 may only refer to a pixel included in thepicture. Such a prediction mode is referred to as intra block copy(IBC). The IBC mode may be used as one mode of normal inter prediction,and may be used with, for example, the merge mode, the normal intermode, the skip mode, and the like. Hereinafter, the IBC mode isdescribed in more detail with reference to the drawings.

FIG. 97 is a diagram for describing the IBC mode. Part (a) of FIG. 97 isa diagram illustrating one example of a region which can be referred toin the IBC mode. Part (b) of FIG. 97 is a diagram illustrating oneexample of a block vector (BV).

In the inter prediction process on a current block, inter predictor 126refers to a block that has been encoded and decoded and is in a picturedifferent from a picture including the current block (that is, a currentpicture). In the prediction process using the IBC mode, however, interpredictor 126 only refers to pixels that have been encoded and decodedand are in the picture including the current block, as in the intraprediction process. For example, as illustrated in part (a) of FIG. 97 ,pixels which can be referred to in a picture are pixels located on theupper or left side of the current block. The region including suchpixels is referred as a “referable region”.

Furthermore, in order to identify a reference block in a frame or apicture that includes the current block, encoder 100 can determine avector that indicates a displacement of the reference block with respectto the current block. This vector is referred to as a block vector (BV).The following describes how the BV is expressed.

As illustrated in part (b) of FIG. 97 , a block vector includes anx-component and a y-component, for example. The x-component (BVx)indicates a horizontal displacement of the reference block with respectto the current block in the picture. The y-component (BVy) indicates avertical displacement of the reference block with respect to the currentblock in the picture. For example, when the origin point (that is, thestarting point) of the BV is the pixel position in the upper left cornerof the current block, the end point of the BV is the pixel position inthe upper left corner of the reference block. In this case, the BV is avector that connects the upper left corner of the current block and theupper left corner of the reference block. Encoder 100 signals the BV inan encoded bitstream to enable decoder 200, when decoding the encodedbitstream, to identify the reference block selected by encoder 100.

[HMVP Mode]

Next, the HMVP mode is described in more detail with reference to FIG.42 again. The MV candidate list (hereinafter, also referred to as amotion vector candidate list) illustrated in the figure may be an MVcandidate list for the merge mode, or may be an MV candidate list forthe normal inter mode.

In the merge mode and the normal inter mode, a motion vector (MV) of thecurrent block is determined by selecting one MV candidate (hereinafter,also referred to as a prediction candidate or a motion vector candidate)from the MV candidate list generated by referring to processed blocks.For example, when generating a prediction image of the current block inthe normal inter mode, encoder 100 selects one MV candidate from the MVcandidate list, takes the difference between an MV of the current blockand the MV candidate, and encodes the difference and an index of the MVcandidate. Furthermore, for example, when generating a prediction imageof the current block in the merge mode, encoder 100 selects one mergeindex from a merge MV candidate list to select an MV candidate and areference picture index that are associated with the merge index.Encoder 100 encodes the selected merge index. Registered in the MVcandidate list are: for example, spatially neighboring motion vectorpredictors (also referred to as spatially neighboring MV candidates)that are motion vectors of encoded blocks that spatially surround thecurrent block; and temporally neighboring motion vector predictors (alsoreferred to as temporally neighboring MV candidates) that are motionvectors of neighboring blocks which are in an encoded reference pictureand on which the position of the current block is projected. MVcandidates in the HMVP mode (hereinafter, also referred to as HMVPmotion vector candidates) are one example of MV candidates registered inthe MV candidate list.

In the HMVP mode, MV candidates are managed using a FIFO buffer for theHMVP mode (hereinafter, also referred as an HMVP table), separately fromthe MV candidate lists for the merge mode and the normal inter mode. Inthe FIFO buffer, a predetermine number (for example, 5 in FIG. 42 ) ofprediction candidates (also referred to as MV candidates) havinginformation related to MVs (hereinafter referred to as MV information)of blocks processed in the past (that is, blocks processed before thecurrent block) are stored in the block processing order, starting fromthe newest one. For example, as illustrated in FIG. 42 , every timeencoder 100 processes one block, encoder 100 stores, in the FIFO buffer,a prediction candidate having MV information of the newest block (inother words, the block processed immediately before the one block). Itis to be noted that every time encoder 100 finishes processing oneblock, encoder 100 may store, in the FIFO buffer, a prediction candidatehaving MV information of the one block.

It is to be noted that the block vector candidate list for the IBC modeis generated using BVs instead of MVs, and the HMVP table stores blockvector candidates (BV candidates) instead of MV candidates. Hereinafter,MVs and BVs are referred to as vectors, MV candidates and BV candidatesare referred to as vector candidates, and the motion vector candidatelist and the block vector candidate list are referred to as vectorcandidate lists. Where appropriate, the MVs and MV candidates may beread as BVs and BV candidates, and the BVs and BV candidates may be readas MVs and MV candidates. It is to be noted that a mixture of BVcandidates and MV candidates may be registered in the motion vectorcandidate list and the block vector candidate list.

When storing in the FIFO buffer a prediction candidate having new MVinformation, encoder 100 deletes from the FIFO buffer a predictioncandidate having MV information of the oldest block (that is, the blockprocessed earliest) in the FIFO buffer. By doing so, encoder 100 canmanage the prediction candidates in the FIFO buffer in the up-to-datecondition. It is to be noted that in the example illustrated in FIG. 42, HMVP1 in the FIFO buffer is the prediction candidate having MVinformation of the newest block, and HMVP5 in the FIFO buffer is theprediction candidate having MV information of the oldest block.

Next, with reference to FIG. 42 , the following describes a pruningprocess in the process of registering MV candidates of the HMVP mode inthe MV candidate lists for the merge mode and the normal inter mode andin the process of updating the FIFO buffer.

[Pruning Process]

The pruning process is a process of comparing an MV which is to beregistered in, for example, a list and an MV registered in, for example,a list and determining whether MV values (or MV information) of theseMVs are the same. In the HMVP mode, there are two types of the pruningprocess: (1) a pruning process for registration in an MV candidate listfrom the HMVP table (this is called a first pruning process) and (2) apruning process for registration of an MV of an immediately previouslyprocessed block (that is, the newest MV) in the HMVP table (that is,update of the HMVP table) (this is called a second pruning process).

Referring to FIG. 42 , for example, in the first pruning processdescribed in (1) above, encoder 100 determines, for each of MVcandidates (for example, HMVP1 through HMVP5) in the FIFO buffer,whether the MV candidate has MV information different from MVinformation of all the MV candidates registered in the MV candidatelist. More specifically, encoder 100 determines, for each of the MVcandidates (HMVP1 through HMVP 5 in this example) in the FIFO buffer,whether the MV information of the MV candidate is different from the MVinformation of all the prediction candidates (that is, MV candidates)registered in the MV candidate list, in an order starting from the MVcandidate having MV information of the newest block (here, HMVP1). Whendetermining that the MV information of HMVP1 is different from the MVinformation of all the MV candidates in the MV candidate list, encoder100 registers HMVP1 in the MV candidate list. Subsequently, encoder 100performs the same process for each of the remaining MV candidates (here,HMVP 2 through HMVP 5) in the FIFO buffer. It is to be noted that thetotal number of HMVP motion vector candidates registered in the MVcandidate list from the FIFO buffer may be 1, or may be 2 or more.

The use of the HMVP mode as described above makes it possible toregister, in the MV candidate list, not only the MV candidates having MVinformation of a block spatially or temporally neighboring the currentblock, but also an MV candidate having MV information of a blockprocessed in the past. This leads to more variations of MV candidatesfor the merge mode and the normal inter mode. Accordingly, encoder 100can select a more appropriate MV candidate for the current block, andthus, the coding efficiency is enhanced.

Also, for example, in the second pruning process described in (2) above,encoder 100 compares an MV of a block immediately previously processed(HMVP0) with all the MV candidates registered in the FIFO buffer (HMVP1through HMVP5 in the figure) to determine whether the MV information ofHMVP0 is different from the MV information of all the MV candidates.When determining that the MV information of HMVP0 is different from theMV information of all the MV candidates in the FIFO buffer, encoder 100deletes the MV of the oldest block in the FIFO buffer (HMVP 5 in thisexample) and registers HMVP0 in the FIFO buffer as the MV of the newestblock. On the other hand, when determining that the MV information ofHMVP0 is identical to the MV information of any one of the MVcandidates, encoder 100 does not register HMVP0 in the FIFO buffer butmaintains the current FIFO buffer (that is, the FIFO buffer in whichHMVP1 through HMVP 5 are registered) as is.

It is to be noted that the MV information may include not only the valueof the MV but also information on a reference picture, information suchas a reference direction and the number of reference pictures, and thelike.

Although the MV candidate lists for the merge mode and the normal intermode have been describe here, the present disclosure is not limited tothis example. For example, in the prediction process of the IBC mode,encoder 100 generates, for each current block, a BV candidate list inwhich BV candidates are registered.

It is to be noted that the MV candidate list and the FIFO bufferillustrated in FIG. 42 are a mere example; the MV candidate list and theFIFO buffer may have a size different from the illustrated size, and mayhave such a configuration in which MV candidates are registered in anorder different from the illustrated order.

Although encoder 100 has been described here as an example, the aboveprocesses are common to both encoder 100 and decoder 200.

[First Aspect]

Now, encoder 100 and decoder 200 according to a first aspect aredescribed.

Encoder 100: in generation of a prediction image of a current block,uses the IBC mode in which an encoded region of a picture that includesthe current block is referred to; determines whether a size of thecurrent block is less than or equal to a threshold, the current blockbeing a unit for which a vector candidate list including vectorcandidates is generated; when the size of the current block is less thanor equal to the threshold, generates the vector candidate list byregistering an HMVP vector candidate in the vector candidate list froman HMVP table without performing a first pruning process, the HMVP tablestoring, by a first-in first-out (FIFO) method, first vector candidateseach as the HMVP vector candidate, the first vector candidates eachhaving information regarding a first vector used for a processed block;when the size of the current block is greater than the threshold,generates the vector candidate list by performing the first pruningprocess and registering the HMVP vector candidate in the vectorcandidate list from the HMVP table; and encodes the current block usingthe vector candidate list.

Also, decoder 200: in generation of a prediction image of a currentblock, uses the IBC mode in which an encoded region of a picture thatincludes the current block is referred to; determines whether a size ofthe current block is less than or equal to a threshold, the currentblock being a unit for which a vector candidate list including vectorcandidates is generated; when the size of the current block is less thanor equal to the threshold, generates the vector candidate list byregistering an HMVP vector candidate in the vector candidate list froman HMVP table without performing a first pruning process, the HMVP tablestoring, by a first-in first-out (FIFO) method, first vector candidateseach as the HMVP vector candidate, the first vector candidates eachhaving information regarding a first vector used for a processed block;when the size of the current block is greater than the threshold,generates the vector candidate list by performing the first pruningprocess and registering the HMVP vector candidate in the vectorcandidate list from the HMVP table; and decodes the current block usingthe vector candidate list.

FIG. 98 is flowchart 1000 illustrating one example of operationsperformed by encoder 100 and decoder 200 according to the first aspect.

In the prediction process for a current picture, encoder 100 starts aloop process which is performed on a block-by-block basis (notillustrated). First, encoder 100 determines the size of a current blockin the IBC mode (S1001). The IBC mode is a prediction mode in which aprocessed region of the picture including the current block is referredto in generation of a prediction image of the current block.

Next, encoder 100 determines whether the size of the current block isless than or equal to a threshold (S1002). For example, the size of thecurrent block may be defined by a total number of pixels included in thecurrent block. In such a case, the threshold is 16 pixels, for example.Also, for example, the size of the current block may be defined by atleast one of the width or the height of the current block. In such acase, the threshold is 4 pixels×4 pixels, for example.

Next, when determining that the size of the current block is not lessthan or equal to the threshold; in other words, when determining thatthe size of the current block is greater than the threshold (NO inS1003), encoder 100 generates a block vector candidate list byperforming the first pruning process and registering an HMVP blockvector candidate in the block vector candidate list (S1004). Forexample, when encoder 100 generates a block vector candidate list forthe IBC mode, encoder 100 performs in step S1004 the first pruningprocess on each of the HMVP block vector candidates in an order startingfrom the newest HMVP block vector candidate, that is, starting from theHMVP block vector candidate that was last entered into the HMVP table tothe HMVP block vector candidate that was first entered into the HMVPtable. More specifically, encoder 100 compares each HMVP block vectorcandidate with all the block vector candidates registered in a table ofthe current block vector candidate list, in an order starting from theHMVP block vector candidate that was last entered into the HMVP table,and determines whether the HMVP block vector candidate is different fromall the block vector candidates. For example, a vector (MV or BV) usedfor a neighboring block may be registered in the block vector candidatelist. For example, when the block vector candidate list includes a blockvector candidate that is identical to the HMVP block vector candidatethat was last entered into the HMVP table, encoder 100 does not add theHMVP block vector candidate to the block vector candidate list. On theother hand, when the block vector candidate list does not include ablock vector candidate identical to the block vector candidate of theHMVP mode that was last entered into the HMVP table, encoder 100 addsthe block vector candidate of the HMVP mode to the block vectorcandidate list. The same process is performed for the other HMVP blockvector candidates stored in the HMVP table. The total number of HMVPblock vector candidates added to the block vector candidate list may be1, or may be 2 or more depending on the total number of vacancies in thelist.

On the other hand, when determining that the size of the current blockis less than or equal to the threshold (YES in S1003), encoder 100generates a block vector candidate list by registering an HMVP blockvector candidate in the block vector candidate list without performingthe first pruning process (S1005). That is to say, encoder 100 in stepS1005 adds the HMVP block vector candidates stored in the HMVP table tothe block vector candidate list in an order starting from the newestone. The total number of HMVP block vector candidates added to the blockvector candidate list may be 1, or may be 2 or more depending on thenumber of vacancies in the list. In such a manner, when the size of thecurrent block is less than or equal to the threshold, encoder 100 skipsthe pruning process, thereby reducing the total number of processingcycles necessary for generating the block vector candidate list. Thisenhances the coding efficiency of encoder 100. Accordingly, in order toreduce the total number of processing cycles necessary for generatingthe block vector candidate list, encoder 100 need not have one blockvector candidate list shared between the current block and a blockneighboring the current block. For example, in the merge mode, encoder100 need not have a merge list shared between the current block and ablock neighboring the current block.

Referring to FIG. 98 again, encoder 100 encodes the current block usingthe block vector candidate list generated in step S1004 or step S1005(S1006).

After repeating the processes of steps S1001 through S1006 for all theblocks included in the current picture, encoder 100 finishes the loopprocess performed on a block-by-block basis (not illustrated).

Although encoder 100 has been described here as an example, the aboveprocesses are common to both encoder 100 and decoder 200.

Here, encoder 100 determines whether to skip the pruning process basedon whether the size of the current block is less than or equal to thethreshold in the generation of a vector candidate list in the IBC mode;however, encoder 100 may perform the same process in the generation of avector candidate list in the merge mode and the normal inter mode aswell. For example, in the generation of a vector candidate list in themerge mode (a merge MV candidate list), encoder 100 determines whetherto skip the pruning process based on whether the size of the currentblock is less than or equal to the threshold. Registered in the merge MVcandidate list are MV candidates and reference picture information(reference picture indices) in association with one another. Thus, evenwhen vector candidates have the same MV value, it is determined in thepruning process that the vector candidates are not identical if thevector candidates have different reference picture information.

Although described here is the example where encoder 100 performs theprediction process using the IBC mode, encoder 100 may perform theprediction process using the IBC mode when determining to use the IBCmode among a plurality of prediction modes, and perform the predictionprocess using a prediction mode different from the IBC mode whendetermining to use the prediction mode different from the IBC mode amongthe plurality of prediction modes. For example, when using a predictionmode different from the IBC mode, encoder 100 may generate a vectorcandidate list by performing the first pruning process and registeringan HMVP vector candidate in the vector candidate list from the HMVPtable, and encode the current block using the vector candidate list.

[Technical Advantages of First Aspect]

The first aspect of the present disclosure introduces a constraintaccording to the size of the current block, into the pruning processperformed on the HMVP vector candidates in the generation of a vectorcandidate list. This enables encoder 100 and decoder 200 to, in thegeneration of a vector candidate list, skip the pruning process ofcomparing the HMVP vector candidates stored in the HMVP table with thevector candidates registered in the vector candidate list when the sizeof the current block is less than or equal to the threshold. As aresult, the total number of cycles necessary for performing the pruningprocess in the generation of a vector candidate list is reduced when thesize of the current block is less than or equal to the threshold.Accordingly, encoder 100 and decoder 200 according to the first aspectbring about reduction in the amount of processing, and thus, the codingefficiency of encoder 100 and the processing efficiency of decoder 200are enhanced.

Although the first aspect has described the example where theabove-described size constraint is introduced into the pruning processperformed when registering an HMVP block vector candidate in the blockvector candidate list from the HMVP table in the generation of the blockvector candidate list for the IBC mode; however, the above-describedsize constraint may also be introduced into the generation of an MVcandidate list for the merge mode and the normal inter mode.

[Second Aspect]

Hereinafter, encoder 100, decoder 200, an encoding method, and adecoding method according to a second aspect are described. While thefirst aspect has described the process of generating a vector candidatelist, the second aspect describes a process of updating the HMVP table.

Encoder 100 according to the second aspect further performs updating ofthe HMVP table using a second vector candidate having informationregarding a second vector. In the update of the HMVP table, encoder 100determines whether the size of the current block is less than or equalto a threshold, updates the HMVP table without performing the secondpruning process when the size of the current block is less than or equalto the threshold, and updates the HMVP table by performing the secondpruning process when the size of the current block is greater than thethreshold.

Decoder 200 according to the second aspect further performs updating ofthe HMVP table using a second vector candidate having informationregarding a second vector. In the update of the HMVP table, decoder 200determines whether the size of the current block is less than or equalto a threshold, updates the HMVP table without performing the secondpruning process when the size of the current block is less than or equalto the threshold, and updates the HMVP table by performing the secondpruning process when the size of the current block is greater than thethreshold.

FIG. 99 is flowchart 2000 illustrating one example of operationsperformed by encoder 100 and decoder 200 according to the second aspect.

In the prediction process for a current picture, encoder 100 starts aloop process which is performed on a block-by-block basis (notillustrated). First, encoder 100 determines the size of a current blockin the IBC mode (S2001).

Next, encoder 100 determines a block vector (the above-mentioned secondvector) of the current block (not illustrated). In the IBC mode, thesecond vector is a vector that indicates the position, in the picture,of a reference block corresponding to the current block.

Next, encoder 100 determines whether the size of the current block isless than or equal to a threshold (S2002). For example, the size of thecurrent block may be defined by the total number of pixels included inthe current block. In such a case, the threshold is 16 pixels, forexample. Also, for example, the size of the current block may be definedby at least one of the width or the height of the current block. In sucha case, the threshold is 4 pixels×4 pixels, for example.

When determining that the size of the current block is not less than orequal to the threshold; in other words, when determining that the sizeof the current block is greater than the threshold (NO in S2003),encoder 100 updates the HMVP table by performing the second pruningprocess (S2004). At this time, encoder 100 updates the HMVP table usinga second block vector candidate having information regarding a secondblock vector. The second block vector is a block vector used forgenerating a prediction image of the current block. For example, in thesecond pruning process, encoder 100 determines whether the second blockvector candidate is identical to any one of first block vectorcandidates (that is, HMVP block vector candidates) in the HMVP table,and when the second block vector candidate is not identical to any oneof the HMVP block vector candidates, updates the HMVP table by storingthe second block vector candidate in the HMVP table. On the other hand,when the second block vector candidate is identical to any one of theHMVP block vector candidates in the HMVP table, encoder 100 discards thesecond block vector candidate. The process of updating the HMVP table byperforming the second pruning process includes the process of discardingthe second block vector candidate. It is to be noted that the thresholdis the same as the threshold described in the first aspect, and thus,the description thereof is omitted.

It is to be noted that as described in the first aspect, in the mergemode and the normal inter mode, a motion vector candidate includes MVinformation having an MV candidate and reference picture information.Thus, when the prediction process is performed on the current block inthe merge mode or the normal inter mode, the second motion vectorcandidate and an HMVP motion vector candidate being identical to eachother in the second pruning process means that the MV information ofthese motion vector candidates are identical. In contrast, when theprediction process is performed on the current block in the IBC mode,the second block vector candidate and an HMVP block vector candidatebeing identical to each other means that the vector values of theseblock vector candidates are identical. More specifically, in step S2004,encoder 100 compares a block vector candidate having informationregarding a block vector that indicates the position of a referenceblock corresponding to the current block in the picture (that is, thesecond block vector candidate) with all the block vector candidatesstored in the HMVP table (that is, the first block vector candidates orthe HMVP block vector candidates), and determines whether the secondblock vector candidate is identical to any one of the HMVP block vectorcandidates. Encoder 100 discards the second block vector candidate whenthe HMVP table includes an HMVP block vector candidate having a vectorvalue identical to that of the second block vector candidate of thecurrent block. In other words, the second block vector candidate is notadded to the HMVP table. On the other hand, encoder 100 adds the secondblock vector candidate to the HMVP table when the HMVP table does notinclude an HMVP block vector candidate having a vector value identicalto that of the second block vector candidate. At this time, the oldestHMVP block vector candidate in the HMVP table is deleted.

Referring to FIG. 99 again, when determining that the size of thecurrent block is less than or equal to the threshold (YES in S2003),encoder 100 updates the HMVP table without performing the second pruningprocess (S2005). For example, encoder 100 updates the HMVP table bystoring in the HMVP table the second block vector candidate havinginformation regarding the second block vector and deleting the oldestHMVP block vector candidate in the HMVP table. More specifically, whenperforming the prediction process on the current block in the IBC mode,encoder 100 in step S2005 adds the block vector candidate havinginformation regarding a block vector of the current block (that is, thesecond block vector candidate) to the HMVP table without performing thepruning process.

Next, encoder 100 encodes the current block after performing the processof updating the HMVP table illustrated in step S2004 and step S2005(step S2006). It is to be noted that the process of updating the HMVPtable may be performed after encoding the current block.

After repeating the processes of steps S2001 through S2006 on all theblocks included in the current picture, encoder 100 finishes the loopprocess performed on a block-by-block basis (not illustrated).

Although encoder 100 has been described here as an example, the aboveprocesses are common to both encoder 100 and decoder 200.

Although the IBC mode has been described here as an example, even in themerge mode and the normal inter mode (for example, the AMVP mode), theHMVP table is used for the generation of the MV candidate list accordingto whether the size of the current block is less than or equal to thethreshold size, as described in the first aspect. Therefore, even in themerge mode and the AMVP mode, the above-described process of updatingthe HMVP table may be performed in the same manner as in the IBC mode.

[Technical Advantages of Second Aspect]

The second aspect of the present disclosure introduces a constraintaccording to the size of the current block, into the pruning processperformed in the update of the HMVP table. This enables encoder 100 anddecoder 200 to, in the update of the HMVP table, skip the pruningprocess of comparing the second vector used for prediction of thecurrent block with all the HMVP vector candidates stored in the HMVPtable when the size of the current block is less than or equal to thethreshold. As a result, the total number of cycles necessary forperforming the pruning process in the update of the HMVP table isreduced when the size of the current block is less than or equal to thethreshold. Accordingly, encoder 100 and decoder 200 according to thesecond aspect bring about reduction in the amount of processing, andthus, the coding efficiency of encoder 100 and the processing efficiencyof decoder 200 are enhanced.

Although the second aspect has described the example of introducing theabove-described size constraint into the pruning process performed whenupdating the HMVP table using the block vector of the current blockpredicted in the IBC mode, the above-described size constraint may beintroduced in the merge mode and the normal inter mode as well.

[Combination with Other Aspects]

One or more aspects disclosed here may be combined with at least aportion of the other aspects of the present disclosure. Moreover, aportion of the processes illustrated in the flow charts, a portion ofthe configurations of the devices, a portion of a syntax, and/or otherfunctions according to one or more aspects disclosed here may becombined with the other aspects.

All the processes/elements described above are not necessarily needed.The device/method may include a portion of the processes/elements. Theprocesses described above may be performed by a decoder as with theencoder.

[Supplementary Notes]

In the ordinary inter prediction, encoder 100 refers to a block includedin a picture different from the current picture; whereas, in the IBCmode, encoder 100 refers to a processed block included in a processedregion in a picture including the current block, that is, the currentpicture. At this time, the region referred to (also referred to as areferable region) is a region which is located in the vicinity of thecurrent block and has been processed relatively recently, and thus, itis a feature of the IBC mode to store the referable region in memory ofa quickly accessible region inside a hardware engine, rather than inmemory outside the hardware engine. As a result, in the predictionprocess of the IBC mode, encoder 100 need not access the memory outsidethe hardware engine; in other words, the memory bandwidth becomesunnecessary. Therefore, encoder 100 only accesses the memory inside thehardware engine, and thus can perform the prediction process at arelatively high speed. Accordingly, in the IBC mode, the current blocksize could take a smaller value than in the other inter predictionmodes. When the current block size is small, however, the number oftimes the process of generating the vector candidate list is performedmay increase, and the amount of processing for the generation of thevector candidate list could become greater than in the other interprediction modes. In addition, the number of times the process ofupdating the HMVP table is performed may also increase, and the amountof processing could become greater than in the other inter predictionmodes. In view of the above, in the case of performing the predictionprocess in the IBC mode, it is possible to reduce the amount ofprocessing by skipping the pruning process in the generation of thevector candidate list and the update of the HMVP table as in the presentembodiment. This could solve the bottleneck of inter predictor 126.

It is to be noted that the technique according to the present embodimentis not limited to the IBC mode, and may also be used in the other interprediction modes in the same manner. The same processes as thosedescribed in the above aspects may be performed in all the interprediction modes used, or may be performed in one or more predictionmodes among all the inter prediction modes used. For example, the sameprocesses may be performed in, among all the inter prediction modes, amode in which the size of the current block could be equal to thethreshold (for example, 16 pixels, that is, =4 pixels×4 pixels).

[Representative Example of Processes]

The following describes a representative example of the processesperformed by above-described encoder 100 and decoder 200.

FIG. 100 is a flow chart illustrating operations performed by encoder100. For example, encoder 100 includes circuitry and memory coupled tothe circuitry. The circuitry and the memory included in encoder 100 maycorrespond to processor a1 and memory a2 illustrated in FIG. 8 . Thecircuitry of encoder 100, in operation, performs the following: Ingeneration of a prediction image of a current block, the circuitry ofencoder 100 uses the IBC mode in which a processed region of a pictureincluding the current block is referred to.

For example, the circuitry of encoder 100 determines whether the size ofthe current block is less than or equal to a threshold (S3001). When thesize of the current block is less than or equal to the threshold (YES inS3001), the circuitry generates a vector candidate list by registeringan HMVP vector candidate in the vector candidate list from the HMVPtable without performing the first pruning process (S3002), and when thesize of the current block is greater than the threshold (NO in S3001),the circuitry generates a vector candidate list by performing the firstpruning process and registering an HMVP vector candidate in the vectorcandidate list from the HMVP table (S3003), and encodes the currentblock using the vector candidate list (S3004).

In the HMVP table, first vector candidates each having informationregarding a first vector used for a processed block are stored as theHMVP vector candidates by the FIFO method, and the vector candidate listis generated for each current block.

Accordingly, encoder 100 generates the vector candidate list withoutperforming the first pruning process (hereinafter referred to as thepruning process) when the size of the current block is less than orequal to the threshold, and thus, the amount of processing is reduced.This enhances the coding efficiency of encoder 100. Moreover, sinceencoder 100 uses the IBC mode in the generation of a prediction image ofthe current block, it is possible to identify a reference block whichhas been encoded and decoded and is in the picture that includes thecurrent block, based on a motion vector indicating the reference block,and read the reference block identified. With this, encoder 100 canidentify, among encoded blocks in the picture that includes the currentblock, a block to be referred to for the current block, based on thevector indicating the position corresponding to the current block, andread the block identified.

For example, the circuitry of encoder 100 may generate a predictionimage of the current block using the second vector, and encode thesecond vector using one of the vector candidates included in the vectorcandidate list. In the first pruning process performed in the generationof the vector candidate list, the circuitry of encoder 100 maydetermine, for each of the HMVP vector candidates stored in the HMVPtable, whether the HMVP vector candidate is identical to any of one ormore vector candidates registered in the vector candidate list. When theHMVP vector candidate is not identical to any of one or more vectorcandidates, the circuitry of encoder 100 may generate the vectorcandidate list by registering the HMVP vector candidate in the vectorcandidate list.

Accordingly, encoder 100 generates the vector candidate list withoutperforming the pruning process when the size of the current block isless than or equal to the threshold, and thus, the amount of processingis reduced. This enhances the coding efficiency of encoder 100.

For example, the circuitry of encoder 100 may further perform update ofthe HMVP table using a second vector candidate having informationregarding a second vector. In the update of the HMVP table, thecircuitry of encoder 100 may determine whether the size of the currentblock is less than or equal to a threshold. When the size of the currentblock is less than or equal to the threshold, the circuitry of encoder100 may update the HMVP table without performing the second pruningprocess. On the other hand, when the size of the current block isgreater than the threshold, the circuitry of encoder 100 may update theHMVP table by performing the second pruning process.

Accordingly, encoder 100 updates the HMVP table without performing thesecond pruning process (hereinafter also referred to as the pruningprocess) when the size of the current block is less than or equal to thethreshold, and thus, the amount of processing is reduced. This enhancesthe coding efficiency of encoder 100.

For example, the circuitry of encoder 100 may determine, in the secondpruning process performed in the update of the HMVP table, whether thesecond vector candidate is identical to any one of the HMVP vectorcandidates. When the second vector candidate is not identical to any oneof the HMVP vector candidates, the circuitry of encoder 100 may updatethe HMVP table by storing the second vector candidate in the HMVP table.

By doing so, encoder 100 can register, in the vector candidate list, avector candidate that is more appropriate for the current block.

For example, the size of the current block may be defined by the totalnumber of pixels included in the current block. For example, thethreshold may be 16 pixels.

Accordingly, encoder 100 generates the vector candidate list and updatesthe HMVP table without performing the pruning process when the totalnumber of pixels included in the current block is less than or equal toa threshold (for example, less than or equal to 16 pixels), and thus,the amount of processing is reduced. This enhances the coding efficiencyof encoder 100.

For example, the size of the current block may be defined by at leastone of the width or the height of the current block. For example, thethreshold may be 4 pixels×4 pixels.

Accordingly, encoder 100 generates the vector candidate list and updatesthe HMVP table without performing the pruning process when at least oneof the width or the height of the current block is less than or equal toa threshold (for example, when both the width and the height of thecurrent block are less than or equal to 4 pixels), and thus, the amountof processing is reduced. This enhances the coding efficiency of encoder100.

For example, the vector candidate list need not be shared between thecurrent block and a block neighboring the current block.

Accordingly, encoder 100 uses the vector candidate lists each generatedfor a different current block, and thus, the prediction precision isenhanced.

For example, in the generation of a prediction image of the currentblock, the circuitry of encoder 100 may use the IBC mode whendetermining to use the IBC mode among the plurality of prediction modes,and use a prediction mode different from the IBC mode when determiningto use the prediction mode different from the IBC mode among theplurality of prediction modes. When using the prediction mode differentfrom the IBC mode, the circuitry of encoder 100 may generate a vectorcandidate list by performing the first pruning process and registeringan HMVP vector candidate in the vector candidate list from the HMVPtable, and encode the current block using the vector candidate list.

This enables encoder 100 to appropriately switch the conditions underwhich the pruning process is performed, according to the prediction modeused. This enhances the coding efficiency of encoder 100.

FIG. 101 is a flow chart illustrating operations performed by decoder200. For example, decoder 200 includes circuitry and memory coupled tothe circuitry. The circuitry and the memory included in decoder 200 maycorrespond to processor b1 and memory b2 illustrated in FIG. 68 . Thecircuitry of decoder 200, in operation, performs the following: Ingeneration of a prediction image of a current block, the circuitry ofdecoder 200 uses the IBC mode in which a processed region of a pictureincluding the current block is referred to.

For example, the circuitry of decoder 200 determines whether the size ofthe current block is less than or equal to a threshold (S4001). When thesize of the current block is less than or equal to the threshold (YES inS4001), the circuitry of decoder 200 generates a vector candidate listby registering an HMVP vector candidate in a vector candidate list fromthe HMVP table without performing the first pruning process (S4002). Onthe other hand, when the size of the current block is greater than thethreshold (NO in S4001), the circuitry of decoder 200 generates a vectorcandidate list by performing the first pruning process and registeringan HMVP vector candidate in the vector candidate list from the HMVPtable (S4003). Subsequently, the circuitry of decoder 200 decodes thecurrent block using the vector candidate list generated in step S4002 orS4003 (S4004).

Accordingly, decoder 200 generates the vector candidate list withoutperforming the pruning process when the size of the current block isless than or equal to the threshold, and thus, the amount of processingis reduced. This enhances the processing efficiency of decoder 200.Moreover, since decoder 200 uses the IBC mode in the generation of aprediction image of the current block, it is possible to identify, amongdecoded blocks in the picture that includes the current block, a blockto be referred to for the current block based on a vector indicating aposition corresponding to the current block, and read the identifiedblock. With this, decoder 200 can identify, among the decoded blocks inthe picture that includes the current block, a block to be referred tofor the current block based on the vector indicating the positioncorresponding to the current block, and read the identified block.

For example, the circuitry of decoder 200 may generate a predictionimage of the current block using the second vector, and decode thesecond vector using one of the vector candidates included in the vectorcandidate list. In the first pruning process performed in the generationof the vector candidate list, the circuitry of decoder 200 maydetermine, for each of the HMVP vector candidates stored in the HMVPtable, whether the HMVP vector candidate is identical to any of one ormore vector candidates registered in the vector candidate list. When theHMVP vector candidate is not identical to any of one or more vectorcandidates, the circuitry of decoder 200 may generate the vectorcandidate list by registering the HMVP vector candidate in the vectorcandidate list.

Accordingly, decoder 200 generates the vector candidate list withoutperforming the pruning process when the size of the current block isless than or equal to the threshold, and thus, the amount of processingis reduced. This enhances the processing efficiency of decoder 200.

For example, the circuitry of decoder 200 may further perform update ofthe HMVP table using a second vector candidate having informationregarding a second vector. In the update of the HMVP table, thecircuitry of decoder 200 may determine whether the size of the currentblock is less than or equal to a threshold. When the size of the currentblock is less than or equal to the threshold, the circuitry of decoder200 may update the HMVP table without performing the second pruningprocess. On the other hand, when the size of the current block isgreater than the threshold, the circuitry of decoder 200 may update theHMVP table by performing the second pruning process.

Accordingly, decoder 200 updates the HMVP table without performing thepruning process when the size of the current block is less than or equalto the threshold, and thus, the amount of processing is reduced. Thisenhances the processing efficiency of decoder 200.

For example, the circuitry of decoder 200 may determine, in the secondpruning process performed in the update of the HMVP table, whether thesecond vector candidate is identical to any one of the HMVP vectorcandidates. When the second vector candidate is not identical to any oneof the HMVP vector candidates, the circuitry of decoder 200 may updatethe HMVP table by storing the second vector candidate in the HMVP table.

By doing so, decoder 200 can register, in the vector candidate list, avector candidate that is more appropriate for the current block.

For example, the size of the current block may be defined by the totalnumber of pixels included in the current block. For example, thethreshold may be 16 pixels.

Accordingly, decoder 200 generates the vector candidate list and updatesthe HMVP table without performing the pruning process when the totalnumber of pixels included in the current block is less than or equal toa threshold (for example, less than or equal to 16 pixels), and thus,the amount of processing is reduced. This enhances the processingefficiency of decoder 200.

For example, the size of the current block may be defined by at leastone of the width or the height of the current block. For example, thethreshold may be 4 pixels×4 pixels.

Accordingly, decoder 200 generates the vector candidate list and updatesthe HMVP table without performing the pruning process when at least oneof the width or the height of the current block is less than or equal toa threshold (for example, when both the width and the height of thecurrent block are less than or equal to 4 pixels), and thus, the amountof processing is reduced. This enhances the processing efficiency ofdecoder 200.

For example, the vector candidate list need not be shared between thecurrent block and a block neighboring the current block.

Accordingly, decoder 200 uses the vector candidate lists each generatedfor a different current block, and thus, the prediction precision isenhanced.

For example, in the generation of a prediction image of the currentblock, the circuitry of decoder 200 may use the IBC mode whendetermining to use the IBC mode among the plurality of prediction modes,and use a prediction mode different from the IBC mode when determiningto use the prediction mode different from the IBC mode among theplurality of prediction modes. When using the prediction mode differentfrom the IBC mode, the circuitry of decoder 200 may generate a vectorcandidate list by performing the first pruning process and registeringan HMVP vector candidate in the vector candidate list from the HMVPtable, and decode the current block using the vector candidate list.

This enables decoder 200 to appropriately switch conditions under whichthe pruning process is performed, according to the prediction mode used.This enhances the processing efficiency of decoder 200.

Each element may be a circuit as described above. These circuits maycollectively form a single circuit, or may be individual circuits. Eachelement may be implemented using a general-purpose processor, or may beimplemented using a dedicated processor.

A process performed by a particular element may be performed by anotherelement. Also, the order in which processes are performed may bechanged, or a plurality of processes may be performed in parallel. Anencoding decoding device may include encoder 100 and decoder 200.

Although aspects of encoder 100 and decoder 200 have been describedabove based on more than one example, the aspects of encoder 100 anddecoder 200 are not limited to such examples. The aspects of encoder 100and decoder 200 may also encompass various modifications to each examplethat are conceivable to a person skilled in the art and embodimentsimplemented by combining elements in different examples, so long as suchmodifications and embodiments do not depart from the essence of thepresent disclosure.

The present aspect may be performed by combining one or more aspectsdisclosed herein with at least part of other aspects according to thepresent disclosure. In addition, the present aspect may be performed bycombining, with the other aspects, part of the processes indicated inany of the flow charts according to the aspects, part of theconfiguration of any of the devices, part of syntaxes, etc.

[Implementations and Applications]

As described in each of the above embodiments, each functional oroperational block may typically be realized as an MPU (micro processingunit) and memory, for example. Moreover, processes performed by each ofthe functional blocks may be realized as a program execution unit, suchas a processor which reads and executes software (a program) recorded ona medium such as ROM. The software may be distributed. The software maybe recorded on a variety of media such as semiconductor memory. Notethat each functional block can also be realized as hardware (dedicatedcircuit).

The processing described in each of the embodiments may be realized viaintegrated processing using a single apparatus (system), and,alternatively, may be realized via decentralized processing using aplurality of apparatuses. Moreover, the processor that executes theabove-described program may be a single processor or a plurality ofprocessors. In other words, integrated processing may be performed, and,alternatively, decentralized processing may be performed.

Embodiments of the present disclosure are not limited to the aboveexemplary embodiments; various modifications may be made to theexemplary embodiments, the results of which are also included within thescope of the embodiments of the present disclosure.

Next, application examples of the moving picture encoding method (imageencoding method) and the moving picture decoding method (image decodingmethod) described in each of the above embodiments will be described, aswell as various systems that implement the application examples. Such asystem may be characterized as including an image encoder that employsthe image encoding method, an image decoder that employs the imagedecoding method, or an image encoder-decoder that includes both theimage encoder and the image decoder. Other configurations of such asystem may be modified on a case-by-case basis.

[Usage Examples]

FIG. 102 illustrates an overall configuration of content providingsystem ex100 suitable for implementing a content distribution service.The area in which the communication service is provided is divided intocells of desired sizes, and base stations ex106, ex107, ex108, ex109,and ex110, which are fixed wireless stations in the illustrated example,are located in respective cells.

In content providing system ex100, devices including computer ex111,gaming device ex112, camera ex113, home appliance ex114, and smartphoneex115 are connected to internet ex101 via internet service providerex102 or communications network ex104 and base stations ex106 throughex110. Content providing system ex100 may combine and connect any of theabove devices. In various implementations, the devices may be directlyor indirectly connected together via a telephone network or near fieldcommunication, rather than via base stations ex106 through ex110.Further, streaming server ex103 may be connected to devices includingcomputer ex111, gaming device ex112, camera ex113, home appliance ex114,and smartphone ex115 via, for example, internet ex101. Streaming serverex103 may also be connected to, for example, a terminal in a hotspot inairplane ex117 via satellite ex116.

Note that instead of base stations ex106 through ex110, wireless accesspoints or hotspots may be used. Streaming server ex103 may be connectedto communications network ex104 directly instead of via internet ex101or internet service provider ex102, and may be connected to airplaneex117 directly instead of via satellite ex116.

Camera ex113 is a device capable of capturing still images and video,such as a digital camera. Smartphone ex115 is a smartphone device,cellular phone, or personal handyphone system (PHS) phone that canoperate under the mobile communications system standards of the 2G, 3G,3.9G, and 4G systems, as well as the next-generation 5G system.

Home appliance ex114 is, for example, a refrigerator or a deviceincluded in a home fuel cell cogeneration system.

In content providing system ex100, a terminal including an image and/orvideo capturing function is capable of, for example, live streaming byconnecting to streaming server ex103 via, for example, base stationex106. When live streaming, a terminal (e.g., computer ex111, gamingdevice ex112, camera ex113, home appliance ex114, smartphone ex115, or aterminal in airplane ex117) may perform the encoding processingdescribed in the above embodiments on still-image or video contentcaptured by a user via the terminal, may multiplex video data obtainedvia the encoding and audio data obtained by encoding audio correspondingto the video, and may transmit the obtained data to streaming serverex103. In other words, the terminal functions as the image encoderaccording to one aspect of the present disclosure.

Streaming server ex103 streams transmitted content data to clients thatrequest the stream. Client examples include computer ex111, gamingdevice ex112, camera ex113, home appliance ex114, smartphone ex115, andterminals inside airplane ex117, which are capable of decoding theabove-described encoded data. Devices that receive the streamed datadecode and reproduce the received data. In other words, the devices mayeach function as the image decoder, according to one aspect of thepresent disclosure.

[Decentralized Processing]

Streaming server ex103 may be realized as a plurality of servers orcomputers between which tasks such as the processing, recording, andstreaming of data are divided. For example, streaming server ex103 maybe realized as a content delivery network (CDN) that streams content viaa network connecting multiple edge servers located throughout the world.In a CDN, an edge server physically near a client is dynamicallyassigned to the client. Content is cached and streamed to the edgeserver to reduce load times. In the event of, for example, some type oferror or change in connectivity due, for example, to a spike in traffic,it is possible to stream data stably at high speeds, since it ispossible to avoid affected parts of the network by, for example,dividing the processing between a plurality of edge servers, orswitching the streaming duties to a different edge server and continuingstreaming.

Decentralization is not limited to just the division of processing forstreaming; the encoding of the captured data may be divided between andperformed by the terminals, on the server side, or both. In one example,in typical encoding, the processing is performed in two loops. The firstloop is for detecting how complicated the image is on a frame-by-frameor scene-by-scene basis, or detecting the encoding load. The second loopis for processing that maintains image quality and improves encodingefficiency. For example, it is possible to reduce the processing load ofthe terminals and improve the quality and encoding efficiency of thecontent by having the terminals perform the first loop of the encodingand having the server side that received the content perform the secondloop of the encoding. In such a case, upon receipt of a decodingrequest, it is possible for the encoded data resulting from the firstloop performed by one terminal to be received and reproduced on anotherterminal in approximately real time. This makes it possible to realizesmooth, real-time streaming.

In another example, camera ex113 or the like extracts a feature amountfrom an image, compresses data related to the feature amount asmetadata, and transmits the compressed metadata to a server. Forexample, the server determines the significance of an object based onthe feature amount and changes the quantization accuracy accordingly toperform compression suitable for the meaning (or content significance)of the image. Feature amount data is particularly effective in improvingthe precision and efficiency of motion vector prediction during thesecond compression pass performed by the server. Moreover, encoding thathas a relatively low processing load, such as variable length coding(VLC), may be handled by the terminal, and encoding that has arelatively high processing load, such as context-adaptive binaryarithmetic coding (CABAC), may be handled by the server.

In yet another example, there are instances in which a plurality ofvideos of approximately the same scene are captured by a plurality ofterminals in, for example, a stadium, shopping mall, or factory. In sucha case, for example, the encoding may be decentralized by dividingprocessing tasks between the plurality of terminals that captured thevideos and, if necessary, other terminals that did not capture thevideos, and the server, on a per-unit basis. The units may be, forexample, groups of pictures (GOP), pictures, or tiles resulting fromdividing a picture. This makes it possible to reduce load times andachieve streaming that is closer to real time.

Since the videos are of approximately the same scene, management and/orinstructions may be carried out by the server so that the videoscaptured by the terminals can be cross-referenced. Moreover, the servermay receive encoded data from the terminals, change the referencerelationship between items of data, or correct or replace picturesthemselves, and then perform the encoding. This makes it possible togenerate a stream with increased quality and efficiency for theindividual items of data.

Furthermore, the server may stream video data after performingtranscoding to convert the encoding format of the video data. Forexample, the server may convert the encoding format from MPEG to VP(e.g., VP9), and may convert H.264 to H.265.

In this way, encoding can be performed by a terminal or one or moreservers. Accordingly, although the device that performs the encoding isreferred to as a “server” or “terminal” in the following description,some or all of the processes performed by the server may be performed bythe terminal, and likewise some or all of the processes performed by theterminal may be performed by the server. This also applies to decodingprocesses.

[3D, Multi-angle]

There has been an increase in usage of images or videos combined fromimages or videos of different scenes concurrently captured, or of thesame scene captured from different angles, by a plurality of terminalssuch as camera ex113 and/or smartphone ex115. Videos captured by theterminals are combined based on, for example, the separately obtainedrelative positional relationship between the terminals, or regions in avideo having matching feature points.

In addition to the encoding of two-dimensional moving pictures, theserver may encode a still image based on scene analysis of a movingpicture, either automatically or at a point in time specified by theuser, and transmit the encoded still image to a reception terminal.Furthermore, when the server can obtain the relative positionalrelationship between the video capturing terminals, in addition totwo-dimensional moving pictures, the server can generatethree-dimensional geometry of a scene based on video of the same scenecaptured from different angles. The server may separately encodethree-dimensional data generated from, for example, a point cloud and,based on a result of recognizing or tracking a person or object usingthree-dimensional data, may select or reconstruct and generate a videoto be transmitted to a reception terminal, from videos captured by aplurality of terminals.

This allows the user to enjoy a scene by freely selecting videoscorresponding to the video capturing terminals, and allows the user toenjoy the content obtained by extracting a video at a selected viewpointfrom three-dimensional data reconstructed from a plurality of images orvideos. Furthermore, as with video, sound may be recorded fromrelatively different angles, and the server may multiplex audio from aspecific angle or space with the corresponding video, and transmit themultiplexed video and audio.

In recent years, content that is a composite of the real world and avirtual world, such as virtual reality (VR) and augmented reality (AR)content, has also become popular. In the case of VR images, the servermay create images from the viewpoints of both the left and right eyes,and perform encoding that tolerates reference between the two viewpointimages, such as multi-view coding (MVC), and, alternatively, may encodethe images as separate streams without referencing. When the images aredecoded as separate streams, the streams may be synchronized whenreproduced, so as to recreate a virtual three-dimensional space inaccordance with the viewpoint of the user.

In the case of AR images, the server superimposes virtual objectinformation existing in a virtual space onto camera informationrepresenting a real-world space, based on a three-dimensional positionor movement from the perspective of the user. The decoder may obtain orstore virtual object information and three-dimensional data, generatetwo-dimensional images based on movement from the perspective of theuser, and then generate superimposed data by seamlessly connecting theimages. Alternatively, the decoder may transmit, to the server, motionfrom the perspective of the user in addition to a request for virtualobject information. The server may generate superimposed data based onthree-dimensional data stored in the server, in accordance with thereceived motion, and encode and stream the generated superimposed datato the decoder. Note that superimposed data includes, in addition to RGBvalues, an a value indicating transparency, and the server sets the avalue for sections other than the object generated fromthree-dimensional data to, for example, 0, and may perform the encodingwhile those sections are transparent. Alternatively, the server may setthe background to a determined RGB value, such as a chroma key, andgenerate data in which areas other than the object are set as thebackground.

Decoding of similarly streamed data may be performed by the client(i.e., the terminals), on the server side, or divided therebetween. Inone example, one terminal may transmit a reception request to a server,the requested content may be received and decoded by another terminal,and a decoded signal may be transmitted to a device having a display. Itis possible to reproduce high image quality data by decentralizingprocessing and appropriately selecting content regardless of theprocessing ability of the communications terminal itself. In yet anotherexample, while a TV, for example, is receiving image data that is largein size, a region of a picture, such as a tile obtained by dividing thepicture, may be decoded and displayed on a personal terminal orterminals of a viewer or viewers of the TV. This makes it possible forthe viewers to share a big-picture view as well as for each viewer tocheck his or her assigned area, or inspect a region in further detail upclose.

In situations in which a plurality of wireless connections are possibleover near, mid, and far distances, indoors or outdoors, it may bepossible to seamlessly receive content using a streaming system standardsuch as MPEG Dynamic Adaptive Streaming over HTTP (MPEG-DASH). The usermay switch between data in real time while freely selecting a decoder ordisplay apparatus including the user's terminal, displays arrangedindoors or outdoors, etc. Moreover, using, for example, information onthe position of the user, decoding can be performed while switchingwhich terminal handles decoding and which terminal handles thedisplaying of content. This makes it possible to map and displayinformation, while the user is on the move in route to a destination, onthe wall of a nearby building in which a device capable of displayingcontent is embedded, or on part of the ground. Moreover, it is alsopossible to switch the bit rate of the received data based on theaccessibility to the encoded data on a network, such as when encodeddata is cached on a server quickly accessible from the receptionterminal, or when encoded data is copied to an edge server in a contentdelivery service.

[Web Page Optimization]

FIG. 103 illustrates an example of a display screen of a web page oncomputer ex111, for example. FIG. 104 illustrates an example of adisplay screen of a web page on smartphone ex115, for example. Asillustrated in FIG. 103 and FIG. 104 , a web page may include aplurality of image links that are links to image content, and theappearance of the web page differs depending on the device used to viewthe web page. When a plurality of image links are viewable on thescreen, until the user explicitly selects an image link, or until theimage link is in the approximate center of the screen or the entireimage link fits in the screen, the display apparatus (decoder) maydisplay, as the image links, still images included in the content or Ipictures; may display video such as an animated gif using a plurality ofstill images or I pictures; or may receive only the base layer, anddecode and display the video.

When an image link is selected by the user, the display apparatusperforms decoding while giving the highest priority to the base layer.Note that if there is information in the Hyper Text Markup Language(HTML) code of the web page indicating that the content is scalable, thedisplay apparatus may decode up to the enhancement layer. Further, inorder to guarantee real-time reproduction, before a selection is made orwhen the bandwidth is severely limited, the display apparatus can reducedelay between the point in time at which the leading picture is decodedand the point in time at which the decoded picture is displayed (thatis, the delay between the start of the decoding of the content to thedisplaying of the content) by decoding and displaying only forwardreference pictures (I picture, P picture, forward reference B picture).Still further, the display apparatus may purposely ignore the referencerelationship between pictures, and coarsely decode all B and P picturesas forward reference pictures, and then perform normal decoding as thenumber of pictures received over time increases.

[Autonomous Driving]

When transmitting and receiving still image or video data such as two-or three-dimensional map information for autonomous driving or assisteddriving of an automobile, the reception terminal may receive, inaddition to image data belonging to one or more layers, information on,for example, the weather or road construction as metadata, and associatethe metadata with the image data upon decoding. Note that metadata maybe assigned per layer and, alternatively, may simply be multiplexed withthe image data.

In such a case, since the automobile, drone, airplane, etc., containingthe reception terminal is mobile, the reception terminal may seamlesslyreceive and perform decoding while switching between base stations amongbase stations ex106 through ex110 by transmitting information indicatingthe position of the reception terminal. Moreover, in accordance with theselection made by the user, the situation of the user, and/or thebandwidth of the connection, the reception terminal may dynamicallyselect to what extent the metadata is received, or to what extent themap information, for example, is updated.

In content providing system ex100, the client may receive, decode, andreproduce, in real time, encoded information transmitted by the user.

[Streaming of Individual Content]

In content providing system ex100, in addition to high image quality,long content distributed by a video distribution entity, unicast ormulticast streaming of low image quality, and short content from anindividual are also possible. Such content from individuals is likely tofurther increase in popularity. The server may first perform editingprocessing on the content before the encoding processing, in order torefine the individual content. This may be achieved using the followingconfiguration, for example.

In real time while capturing video or image content, or after thecontent has been captured and accumulated, the server performsrecognition processing based on the raw data or encoded data, such ascapture error processing, scene search processing, meaning analysis,and/or object detection processing. Then, based on the result of therecognition processing, the server—either when prompted orautomatically—edits the content, examples of which include: correctionsuch as focus and/or motion blur correction; removing low-priorityscenes such as scenes that are low in brightness compared to otherpictures, or out of focus; object edge adjustment; and color toneadjustment. The server encodes the edited data based on the result ofthe editing. It is known that excessively long videos tend to receivefewer views. Accordingly, in order to keep the content within a specificlength that scales with the length of the original video, the servermay, in addition to the low-priority scenes described above,automatically clip out scenes with low movement, based on an imageprocessing result. Alternatively, the server may generate and encode avideo digest based on a result of an analysis of the meaning of a scene.

There may be instances in which individual content may include contentthat infringes a copyright, moral right, portrait rights, etc. Suchinstance may lead to an unfavorable situation for the creator, such aswhen content is shared beyond the scope intended by the creator.Accordingly, before encoding, the server may, for example, edit imagesso as to blur faces of people in the periphery of the screen or blur theinside of a house, for example. Further, the server may be configured torecognize the faces of people other than a registered person in imagesto be encoded, and when such faces appear in an image, may apply amosaic filter, for example, to the face of the person. Alternatively, aspre- or post-processing for encoding, the user may specify, forcopyright reasons, a region of an image including a person or a regionof the background to be processed. The server may process the specifiedregion by, for example, replacing the region with a different image, orblurring the region. If the region includes a person, the person may betracked in the moving picture, and the person's head region may bereplaced with another image as the person moves.

Since there is a demand for real-time viewing of content produced byindividuals, which tends to be small in data size, the decoder firstreceives the base layer as the highest priority, and performs decodingand reproduction, although this may differ depending on bandwidth. Whenthe content is reproduced two or more times, such as when the decoderreceives the enhancement layer during decoding and reproduction of thebase layer, and loops the reproduction, the decoder may reproduce a highimage quality video including the enhancement layer. If the stream isencoded using such scalable encoding, the video may be low quality whenin an unselected state or at the start of the video, but it can offer anexperience in which the image quality of the stream progressivelyincreases in an intelligent manner. This is not limited to just scalableencoding; the same experience can be offered by configuring a singlestream from a low quality stream reproduced for the first time and asecond stream encoded using the first stream as a reference.

[Other Implementation and Application Examples]

The encoding and decoding may be performed by LSI (large scaleintegration circuitry) ex500 (see FIG. 102 ), which is typicallyincluded in each terminal. LSI ex500 may be configured of a single chipor a plurality of chips. Software for encoding and decoding movingpictures may be integrated into some type of a medium (such as a CD-ROM,a flexible disk, or a hard disk) that is readable by, for example,computer ex111, and the encoding and decoding may be performed using thesoftware. Furthermore, when smartphone ex115 is equipped with a camera,video data obtained by the camera may be transmitted. In this case, thevideo data is coded by LSI ex500 included in smartphone ex115.

Note that LSI ex500 may be configured to download and activate anapplication. In such a case, the terminal first determines whether it iscompatible with the scheme used to encode the content, or whether it iscapable of executing a specific service. When the terminal is notcompatible with the encoding scheme of the content, or when the terminalis not capable of executing a specific service, the terminal firstdownloads a codec or application software and then obtains andreproduces the content.

Aside from the example of content providing system ex100 that usesinternet ex101, at least the moving picture encoder (image encoder) orthe moving picture decoder (image decoder) described in the aboveembodiments may be implemented in a digital broadcasting system. Thesame encoding processing and decoding processing may be applied totransmit and receive broadcast radio waves superimposed with multiplexedaudio and video data using, for example, a satellite, even though thisis geared toward multicast, whereas unicast is easier with contentproviding system ex100.

[Hardware Configuration]

FIG. 105 illustrates further details of smartphone ex115 shown in FIG.102 . FIG. 106 illustrates a configuration example of smartphone ex115.Smartphone ex115 includes antenna ex450 for transmitting and receivingradio waves to and from base station ex110, camera ex465 capable ofcapturing video and still images, and display ex458 that displaysdecoded data, such as video captured by camera ex465 and video receivedby antenna ex450. Smartphone ex115 further includes user interface ex466such as a touch panel, audio output unit ex457 such as a speaker foroutputting speech or other audio, audio input unit ex456 such as amicrophone for audio input, memory ex467 capable of storing decoded datasuch as captured video or still images, recorded audio, received videoor still images, and mail, as well as decoded data, and slot ex464 whichis an interface for Subscriber Identity Module (SIM) ex468 forauthorizing access to a network and various data. Note that externalmemory may be used instead of memory ex467.

Main controller ex460, which comprehensively controls display ex458 anduser interface ex466, power supply circuit ex461, user interface inputcontroller ex462, video signal processor ex455, camera interface ex463,display controller ex459, modulator/demodulator ex452,multiplexer/demultiplexer ex453, audio signal processor ex454, slotex464, and memory ex467 are connected via bus ex470.

When the user turns on the power button of power supply circuit ex461,smartphone ex115 is powered on into an operable state, and eachcomponent is supplied with power from a battery pack.

Smartphone ex115 performs processing for, for example, calling and datatransmission, based on control performed by main controller ex460, whichincludes a CPU, ROM, and RAM. When making calls, an audio signalrecorded by audio input unit ex456 is converted into a digital audiosignal by audio signal processor ex454, to which spread spectrumprocessing is applied by modulator/demodulator ex452 and digital-analogconversion and frequency conversion processing are applied bytransmitter/receiver ex451, and the resulting signal is transmitted viaantenna ex450. The received data is amplified, frequency converted, andanalog-digital converted, inverse spread spectrum processed bymodulator/demodulator ex452, converted into an analog audio signal byaudio signal processor ex454, and then output from audio output unitex457. In data transmission mode, text, still-image, or video data istransmitted by main controller ex460 via user interface input controllerex462 based on operation of user interface ex466 of the main body, forexample. Similar transmission and reception processing is performed. Indata transmission mode, when sending a video, still image, or video andaudio, video signal processor ex455 compression encodes, by the movingpicture encoding method described in the above embodiments, a videosignal stored in memory ex467 or a video signal input from camera ex465,and transmits the encoded video data to multiplexer/demultiplexer ex453.Audio signal processor ex454 encodes an audio signal recorded by audioinput unit ex456 while camera ex465 is capturing a video or still image,and transmits the encoded audio data to multiplexer/demultiplexer ex453.Multiplexer/demultiplexer ex453 multiplexes the encoded video data andencoded audio data using a determined scheme, modulates and converts thedata using modulator/demodulator (modulator/demodulator circuit) ex452and transmitter/receiver ex451, and transmits the result via antennaex450.

When a video appended in an email or a chat, or a video linked from aweb page, is received, for example, in order to decode the multiplexeddata received via antenna ex450, multiplexer/demultiplexer ex453demultiplexes the multiplexed data to divide the multiplexed data into abitstream of video data and a bitstream of audio data, supplies theencoded video data to video signal processor ex455 via synchronous busex470, and supplies the encoded audio data to audio signal processorex454 via synchronous bus ex470. Video signal processor ex455 decodesthe video signal using a moving picture decoding method corresponding tothe moving picture encoding method described in the above embodiments,and video or a still image included in the linked moving picture file isdisplayed on display ex458 via display controller ex459. Audio signalprocessor ex454 decodes the audio signal and outputs audio from audiooutput unit ex457. Since real-time streaming is becoming increasinglypopular, there may be instances in which reproduction of the audio maybe socially inappropriate, depending on the user's environment.Accordingly, as an initial value, a configuration in which only videodata is reproduced, i.e., the audio signal is not reproduced, may bepreferable; and audio may be synchronized and reproduced only when aninput is received from the user clicking video data, for instance.

Although smartphone ex115 was used in the above example, three otherimplementations are conceivable: a transceiver terminal including bothan encoder and a decoder; a transmitter terminal including only anencoder; and a receiver terminal including only a decoder. In thedescription of the digital broadcasting system, an example is given inwhich multiplexed data obtained as a result of video data beingmultiplexed with audio data is received or transmitted. The multiplexeddata, however, may be video data multiplexed with data other than audiodata, such as text data related to the video. Further, the video dataitself rather than multiplexed data may be received or transmitted.

Although main controller ex460 including a CPU is described ascontrolling the encoding or decoding processes, various terminals ofteninclude Graphics Processing Units (GPUs). Accordingly, a configurationis acceptable in which a large area is processed at once by making useof the performance ability of the GPU via memory shared by the CPU andGPU, or memory including an address that is managed so as to allowcommon usage by the CPU and GPU. This makes it possible to shortenencoding time, maintain the real-time nature of streaming, and reducedelay. In particular, processing relating to motion estimation,deblocking filtering, sample adaptive offset (SAO), andtransformation/quantization can be effectively carried out by the GPU,instead of the CPU, in units of pictures, for example, all at once.

Although only some exemplary embodiments of the present disclosure havebeen described in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of the present disclosure. Accordingly, all suchmodifications are intended to be included within the scope of thepresent disclosure.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to, for example, televisionreceivers, digital video recorders, car navigation systems, mobilephones, digital cameras, digital video cameras, teleconferencingsystems, electronic mirrors, etc.

What is claimed is:
 1. An encoder comprising: circuitry; and memorycoupled to the circuitry, wherein the circuitry, in operation: ingeneration of a prediction image of a current block, uses an intra blockcopy (IBC) mode in which a processed region of a picture that includesthe current block is referred to; determines whether a size of thecurrent block is less than or equal to a threshold, the current blockbeing a unit for which a vector candidate list including vectorcandidates is generated; when the size of the current block is less thanor equal to the threshold, generates the vector candidate list byregistering a history-based motion vector predictor (HMVP) vectorcandidate in the vector candidate list from an HMVP table withoutperforming a first pruning process, the HMVP table storing, by afirst-in first-out (FIFO) method, first vector candidates each as theHMVP vector candidate, the first vector candidates each havinginformation regarding a first vector used for a processed block; whenthe size of the current block is greater than the threshold, generatesthe vector candidate list by performing the first pruning process andregistering the HMVP vector candidate in the vector candidate list fromthe HMVP table; and encodes the current block using the vector candidatelist, and wherein the first pruning process is a process of comparingthe HMVP vector candidate stored in the HMVP table and a vectorcandidate registered in the vector candidate list, and determiningwhether the HMVP vector candidate and the vector candidate are the same.2. A decoder comprising: circuitry; and memory coupled to the circuitry,wherein the circuitry, in operation: in generation of a prediction imageof a current block, uses an intra block copy (IBC) mode in which aprocessed region of a picture that includes the current block isreferred to; determines whether a size of the current block is less thanor equal to a threshold, the current block being a unit for which avector candidate list including vector candidates is generated; when thesize of the current block is less than or equal to the threshold,generates the vector candidate list by registering a history-basedmotion vector predictor (HMVP) vector candidate in the vector candidatelist from an HMVP table without performing a first pruning process, theHMVP table storing, by a first-in first-out (FIFO) method, first vectorcandidates each as the HMVP vector candidate, the first vectorcandidates each having information regarding a first vector used for aprocessed block; when the size of the current block is greater than thethreshold, generates the vector candidate list by performing the firstpruning process and registering the HMVP vector candidate in the vectorcandidate list from the HMVP table; and decodes the current block usingthe vector candidate list, and wherein the first pruning process is aprocess of comparing the HMVP vector candidate stored in the HMVP tableand a vector candidate registered in the vector candidate list, anddetermining whether the HMVP vector candidate and the vector candidateare the same.
 3. An encoding method comprising: in generation of aprediction image of a current block, using an intra block copy (IBC)mode in which a processed region of a picture that includes the currentblock is referred to; determining whether a size of the current block isless than or equal to a threshold, the current block being a unit forwhich a vector candidate list including vector candidates is generated;when the size of the current block is less than or equal to thethreshold, generating the vector candidate list by registering ahistory-based motion vector predictor (HMVP) vector candidate in thevector candidate list from an HMVP table without performing a firstpruning process, the HMVP table storing, by a first-in first-out (FIFO)method, first vector candidates each as the HMVP vector candidate, thefirst vector candidates each having information regarding a first vectorused for a processed block; when the size of the current block isgreater than the threshold, generating the vector candidate list byperforming the first pruning process and registering the HMVP vectorcandidate in the vector candidate list from the HMVP table; and encodingthe current block using the vector candidate list, wherein the firstpruning process is a process of comparing the HMVP vector candidatestored in the HMVP table and a vector candidate registered in the vectorcandidate list, and determining whether the HMVP vector candidate andthe vector candidate are the same.
 4. A decoding method comprising: ingeneration of a prediction image of a current block, using an intrablock copy (IBC) mode in which a processed region of a picture thatincludes the current block is referred to; determining whether a size ofthe current block is less than or equal to a threshold, the currentblock being a unit for which a vector candidate list including vectorcandidates is generated; when the size of the current block is less thanor equal to the threshold, generating the vector candidate list byregistering a history-based motion vector predictor (HMVP) vectorcandidate in the vector candidate list from an HMVP table withoutperforming a first pruning process, the HMVP table storing, by afirst-in first-out (FIFO) method, first vector candidates each as theHMVP vector candidate, the first vector candidates each havinginformation regarding a first vector used for a processed block; whenthe size of the current block is greater than the threshold, generatingthe vector candidate list by performing the first pruning process andregistering the HMVP vector candidate in the vector candidate list fromthe HMVP table; and decoding the current block using the vectorcandidate list, wherein the first pruning process is a process ofcomparing the HMVP vector candidate stored in the HMVP table and avector candidate registered in the vector candidate list, anddetermining whether the HMVP vector candidate and the vector candidateare the same.